An ankle sprain occurs when the joint’s ligament is stretched or torn. Ligaments are bands or sheets of regular, tough fibrous tissue that connect bones together. Symptoms of an ankle sprain include swelling and discoloration near the affected area. Ankle sprains may be classified as follows:

• Type I sprain – ligaments stretched

• Type II sprain – ligaments slightly torn

• Type III sprain – ligaments completely torn

Treatment for a Type I sprain should include rest, ice, compression and immobilization, and elevation of the affected area. This is easy to remember if you think of the acronym RICE. If you suspect a ligament is torn or completely severed, see your medical care professional for treatment.

The nervous system enables a person to blink to prevent harmful substances from getting in the eyes. During the normal course of a day, a person blinks an average of 15 times a minute to keep the eyes healthy. The lacrimal gland provides lubricating fluid for the eyes. The eyelid moves fluid from the lacrimal gland and across the eye. Blinking also provides the eyes with protection from foreign objects.

When the eye becomes irritated, the lacrimal gland produces extra tears to wash out impurities. Excess fluid drains through the tear ducts and into the nasal cavity. An abundance of tears draining through the nasal cavity may cause the nose to run and a person to sniffle.

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The brain is composed of more than a thousand million neurons. Specific groups of them, working in concert, provide us with the capacity to reason, to experience feelings, and to understand the world. They also give us the capacity to remember numerous pieces of information.

The 3 major components of the brain are the cerebrum, cerebellum, and brain stem.

The cerebrum is divided into is left and right hemispheres, each composed of a frontal, temporal, parietal, and occipital lobes. The cerebral cortex (gray matter) is the outside portion of the cerebrum and provides us with functions associated with conscious thought. The grooves and folds increase the cerebrum’s surface area, allowing us to have a tremendous amount of gray matter inside of the skull. Deep to the gray matter is the cerebral "white matter". The white matter provides for the communication between the cortex and lower central nervous system centers.

The cerebellum is located near the base of the head. It creates automatic programs so we can make complex movements without thinking.

The brain stem connects the brain with the spinal cord and is composed of 3 structures: the midbrain, pons, and medulla oblongata. The brain stem provides us with automatic functions that are necessary for survival.

The two lungs are the primary organs of the respiratory system. Other components of the respiratory system conduct air to the lungs, such as the trachea (windpipe) which branches into smaller structures called bronchi.

The process of breathing (respiration) is divided into two distinct phases, inspiration (inhalation) and expiration (exhalation). During inspiration, the diaphragm contracts and pulls downward while the muscles between the ribs contract and pull upward. This increases the size of the thoracic cavity and decreases the pressure inside. As a result, air rushes in and fills the lungs.

During expiration, the diaphragm relaxes, and the volume of the thoracic cavity decreases, while the pressure within it increases. As a result, the lungs contract and air is forced out.

During intercourse, sperm are released into the vagina near the cervix, swim through the uterus and travel up the fallopian tubes. Sperm are composed of 3 parts: a head, a middle section, and a tail. The tail propels the sperm, which is powered by energy cells stored in the middle section. The head of the sperm contains the man’s genetic material and an enzyme-filled acrosomal cap needed to help the sperm penetrate through the outer membrane of the egg.

As an egg released by an ovary travels through a fallopian tube, it may encounter hundreds of sperm that have survived to reach this point in their journey. Eventually, one sperm may succeed in breaking through the egg’s outer membrane.

After penetrating the egg’s outer membrane, the sperm releases its nucleus, which unites with the nucleus from the egg. Fertilization or conception occurs when the sperm fuses with the egg to form a fertilized egg (zygote).

Coronary artery bypass graft surgery (CABG) is an invasive procedure that involves taking a section of vein from the leg and grafting it onto a location on the heart, which allows blood to bypass the blocked portion of the coronary artery.

The procedure begins with the surgeon making a cut in the leg and removing a section of vein. Both ends of the vein are tied-off in the leg and cut is closed. The chest is opened and the blood is rerouted through a heart-lung machine. The heart is then stopped.

The surgeon locates the blocked coronary artery and attaches the section of vein taken from the leg to the aorta and to the coronary artery below the blocked segment of the artery. The surgeon may do as many bypasses on as many blocked coronary arteries as the patient needs.

Once each bypass graft is placed, it is checked for leaks. Following this, the heart is restarted. Once the heart is beating again, the surgeon will remove its attachments to the heart-lung machine and sew the openings closed. Following this the chest is closed. A pacemaker may be inserted during the procedure to help control any heart rhythm problems the patient may have.

Coughing is a sudden expulsion of air from the lungs through the epiglottis at an amazingly fast speed (estimated at 100 miles per hour). With such a strong force of air, coughing is the body’s mechanism for clearing the breathing passageways of unwanted irritants.

In order for a cough to occur, several events need to take place in sequence. First, the vocal cords open widely, allowing additional air to pass through into the lungs. Then the epiglottis closes off the windpipe (larynx), and simultaneously, the abdominal and rib muscles contract, increasing the pressure behind the epiglottis. With the increased pressure, the air is forcefully expelled, and creates a rushing sound as it moves very quickly past the vocal cords. The rushing air dislodges the irritant, making it possible to breathe comfortably again.

Digestion is the process in which food is broken down into nutrients used by the body. Food passes from the mouth through the esophagus to the stomach. The stomach churns the food and breaks it down further with its contents of hydrochloric acid and an enzyme called pepsin.

The process of breaking food down in the stomach takes a few hours. From there, it goes to the duodenum where it is broken down further by digestive bile produced by the liver and stored in the gallbladder along with enzymes from the pancreas. Enzymes are chemicals that speed up the digestion of specific types of food. For example, the enzyme trypsin breaks down the protein in steak, lipase helps to break down fat, and lactase breaks down the sugar in milk.

Once everything is broken down, the small intestine absorbs the nutrients the body needs. From there the nutrients go into the bloodstream and to the liver, where poisons are removed. Undigested food and water continue through the small intestine and go into the large intestine, where water is reabsorbed. Finally, feces are eliminated through the rectum and anus.

Directional Coronary Atherectomy (DCA) is a minimally invasive procedure to remove the blockage from the coronary arteries and allow more blood to flow to the heart muscle and ease the pain caused by blockages.

The procedure begins with the doctor injecting some local anesthesia into the groin area and putting a needle into the femoral artery, the blood vessel that runs down the leg. A guide wire is placed through the needle and the needle is removed. An introducer is then placed over the guide wire, after which the wire is removed. A different sized guide wire is put in its place.

Next, a long narrow tube called a diagnostic catheter is advanced through the introducer over the guide wire, into the blood vessel. This catheter is then guided to the aorta and the guide wire is removed. Once the catheter is placed in the opening or ostium of one the coronary arteries, the doctor injects dye and takes an x-ray.

If a treatable blockage is noted, the first catheter is exchanged for a guiding catheter. Once the guiding catheter is in place, a guide wire is advanced across the blockage, then a catheter designed for lesion cutting is advanced across the blockage site. A low-pressure balloon, which is attached to the catheter adjacent to the cutter, is inflated such that the lesion material is exposed to the cutter.

The cutter spins, cutting away pieces of the blockage. These lesion pieces are stored in a section of the catheter called a nosecone, and removed after the intervention is complete. Together with rotation of the catheter, the balloon can be deflated and re-inflated to cut the blockage in any direction, allowing for uniform debulking.

A device called a stent may be placed within the coronary artery to keep the vessel open. After the intervention is completed the doctor injects contrast media and takes an x-ray to check for any change in the arteries. Following this, the catheter is removed and the procedure is completed.

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A woman is born with all of the egg cells she will release throughout her lifetime. Starting at about age 12 through menopause, a woman’s reproductive cycle releases an egg about once a month.

Hormonal messages from the brain instruct the ovaries to develop several follicles in which a single dominant follicle in one of the ovaries will release an egg for fertilization. During this time, other hormones instruct the uterine lining to thicken in preparation for nourishing a fertilized egg.

There are several hormones that regulate the reproductive cycle. Follicle stimulating hormone (FSH) stimulates preparation of the egg for fertilization by instructing a follicle to begin dividing it’s genetic material (chromosomes).

The follicle then releases estrogen, the hormone that prepares the lining of the uterus to receive a fertilized egg. Increased levels of estrogen in the bloodstream cause a small structure in the brain, the pituitary gland, to stop releasing the hormone FSH, and to start releasing luteinizing hormone (LH).

LH causes the follicle to enlarge rapidly and to release its egg in a process known as ovulation. Once the egg is out of the follicle, the follicle begins secreting the hormone progesterone, which also helps to prepare the uterine lining for the fertilized egg. The remaining cells of the follicle shrink into a hormone producing mass of cells called a corpus luteum.

The egg is swept into the fallopian tube by its waving structures called fimbriae. Fertilization of the egg usually occurs in the fallopian tube. From there, it is transported to the uterus and implants itself in the uterine wall, where it is nourished by the uterine lining. In the ovary, the corpus luteum produces progesterone so that the egg can develop into a fetus.

If the egg is not fertilized within 24 hours after its release from the ovary, it stops developing and dissolves before reaching the uterus. The absence of a fertilized egg causes the body to stop releasing the hormones that prepare the uterus for implantation. In response, the uterus sheds its lining over a period of four to five days in a process known as menstruation.

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The ears begin their development during the fifth week of pregnancy. Ear formation starts from a few small bulges called branchial arches. Portions of the branchial arches form into structures called auricular hillocks. The auricular hillocks grow and join together to form the outer ears.

During the fifth month, the inner and middle parts of the ear develop, but won’t be completely finished until birth.

Twins occur in about 1% of all pregnancies in which 30% are identical (maternal, monozygotic) twins and 70% are non-identical (fraternal, dizygotic) twins.

A single baby is formed when an egg cell is fertilized by a single sperm cell to form a zygote. The zygote divides to form a structure composed of hundreds of cells called a blastocyst. The blastocyst implants into the uterine lining and will grow into a single baby.

Identical twins start out from a single fertilized egg cell (zygote). Unlike a single baby, the fertilized egg cell will split into two separate embryos during the two-cell stage (day 2), early blastocyst stage (day 4), or late blastocyst stage (day 6).

The stage at which the egg cell splits determines how the twins will implant in the uterine lining, and whether or not they share an amnion, chorion, and placenta. The earlier the splitting occurs, the more independently the twins will develop in the uterus. Twins that split during the late blastocyst stage will share an amnion, chorion, and amniotic sac.

Non-identical twins develop from two fertilized egg cells (zygotes). During ovulation, two egg cells are released and fertilized by two different sperm cells. Non-identical twin embryos develop separately each having their own chorion, amnion, and placenta.

Air first enters the body through the mouth or nose, quickly moves to the pharynx (throat), passes through the larynx (voice box), enters the trachea, which branches into a left and right bronchus within the lungs and further divides into smaller and smaller branches called bronchioles. The smallest bronchioles end in tiny air sacs, called alveoli, which inflate during inhalation, and deflate during exhalation.

Gas exchange is the delivery of oxygen from the lungs to the bloodstream, and the elimination of carbon dioxide from the bloodstream to the lungs. It occurs in the lungs between the alveoli and a network of tiny blood vessels called capillaries, which are located in the walls of the alveoli.

The walls of the alveoli actually share a membrane with the capillaries in which oxygen and carbon dioxide to move freely between the respiratory system and the bloodstream. Oxygen molecules attach to red blood cells, which travel back to the heart. At the same time, the carbon dioxide molecules in the alveoli are blown out of the body with the next exhalation.

The ear is divided into three regions: the outer ear, middle ear and inner ear.

When sound waves enter the ear canal, they cause the eardrum to vibrate. The vibration moves the three bones in the middle ear, called the ossicles. The ossicles are also known as the hammer (malleus), anvil (incus), and stirrup (stapes). These tiny bones transfer and amplify sound waves to the oval window, which is located behind the stirrup.

When the oval window vibrates, it moves fluid across a membrane inside the cochlea. The fluid causes the membrane to move. Specialized hair cells translate this movement into nerve impulses, which are sent to the brain through the vestibulocochlear nerve. The brain interprets the impulses as sound.

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The urinary tract includes the kidneys, ureters, bladder and urethra. Within each kidney, urine flows from the outer cortex to the inner medulla. The renal pelvis is the funnel through which urine exits the kidney and enters the ureter.

As urine can become very concentrated as it passes through the kidneys. When the urine becomes too concentrated, calcium, uric acid salts and other chemicals dissolved in the urine can crystallize, forming a kidney stone (renal calculus).

Usually the calculus is the size of a small pebble. But ureters are very sensitive to being stretched, and when stones form and distend it, the stretching can be very painful. Often, people may not know they have kidney stones until they feel the painful symptoms resulting from a stone being stuck anywhere along the urinary tract. Fortunately, small stones typically passed out of the kidneys and through the ureters on their own without causing any problems.

However, stones can become more problematic when they block the flow of urine. A staghorn kidney stone may obstruct the entire kidney. Fortunately, these stones are the exception rather than the rule.

At 1 month, the baby growing inside the mother’s uterus is very small. The baby is so small she could fit in the palm of your hand and is about the size of your thumbnail.

Over the next 9 months, the baby will grow more inside the uterus until she is ready to be born.

To make a baby, a man’s sperm meets and joins with a woman’s egg cell inside her body. Inside the man’s sperm are a set of instructions that tell the baby to be a boy or a girl.

The instructions in the man’s sperm cell can either carry the letter "X" or the letter "Y". If the letter is an "X", it means the baby will be a girl. If the letter is a "Y", the baby will be a boy.

When the baby is in the mother’s uterus, it can’t eat or breathe on its own, so it needs some help. The baby has a little tube that goes to its middle called the umbilical cord. The umbilical cord goes to the placenta, which connects to the mother’s uterus.

Here’s how it works. First, the food that the mother eats and air that she breathes get into her bloodstream as very tiny pieces called molecules.

These molecules, or tiny pieces of food and air, travel through the mother’s bloodstream to her placenta. From there, they go to the umbilical cord and into the baby’s body. That’s how the baby eats and breathes inside the uterus.

After a baby is born, the umbilical cord goes away. Guess what’s left? You’re belly button.

Two things are needed to make a baby: a sperm cell and an egg cell. A man makes the sperm cell inside his body and a woman makes the egg cell inside her body.

Both the sperm cell and egg cell are very small. You would need a microscope to see them in real life. A microscope is like a magnifying glass, only much stronger.

When the sperm cell and the egg cell meet each other, they make a tiny baby that’s smaller than a grain of salt. The baby will grow inside a special place in woman’s body called the uterus. After about nine months, the baby will come out as a little boy or girl.

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Muscles perform four important body functions: maintain body posture, stabilize the joints, provide mobility, and generate heat that the body requires.

The body contains three types of muscle to perform these functions:
• Smooth muscle - involuntary muscle found in the walls of body organs; functions without conscious control
• Cardiac muscle - involuntary muscle found only in the walls of the heart; functions without conscious control
• Skeletal muscle - attaches to and covers the bony skeleton to provide movement of the body; the only type of muscle under voluntary or conscious control

The nervous system is composed of two divisions, the central nervous system (CNS) and peripheral nervous system (PNS). The CNS contains the brain and the spinal cord and the PNS consists of thousands of nerves that connect the spinal cord to muscles and sensory receptors.

A peripheral nerve is composed of nerve bundles (fascicles) that contain hundreds of individual nerve fibers (neurons). Neurons consist of dendrites, axon, and cell body. The dendrites are the tree-like structures that receive signals from other neurons and from special sensory cells that sense the body’s surrounding environment. The cell body is the headquarters of the neuron and contains its genetic information in the form of DNA. The axon transmits signals away from the cell body to other neurons.

Many neurons are insulated like pieces of electrical wire. This insulation protects them and also allows their signals to move faster along the axon. Without this insulation, signals from the brain might never reach the outlying muscle groups in the limbs.

The operation of the nervous system depends on the flow of communication between neurons. For an electrical signal to travel between two neurons, it must first be converted to a chemical signal, which then crosses a space of about a millionth of an inch wide. The space is called a synapse, and the chemical signal is called a neurotransmitter.

Neurotransmitters allow the billions of neurons in the nervous system to communicate with one another, making the nervous system the master communication system of the body.

Osteoarthritis is the most common form of arthritis and is associated with the aging process. Osteoarthritis is a chronic disease causing the deterioration of the cartilage within a joint.

For most people, the cause of osteoarthritis is unknown, but metabolic, genetic, chemical, and mechanical factors play a role in its development. Symptoms of osteoarthritis include loss of flexibility, limited movement, and pain and swelling within the joint. The condition results from injury to the cartilage, which normally absorbs stress and covers the bones, so they can move smoothly.

The cartilage of the affected joint is roughened and becomes worn down. As the disease progresses, the cartilage becomes completely worn down and the bone rubs on bone. Bony spurs usually develop around the margins of the joint.
Part of the pain results from these bone spurs, which can restrict the joint’s movement as well.

Percutaneous Transluminal Coronary Angioplasty (PTCA) is a minimally invasive procedure to open up blocked coronary arteries, allowing blood to circulate unobstructed to the heart muscle.

The procedure begins with the doctor injecting some local anesthesia into the groin area and putting a needle into the femoral artery, the blood vessel that runs down the leg. A guide wire is placed through the needle and the needle is removed. An introducer is then placed over the guide wire, after which the wire is removed. A different sized guide wire is put in its place.

Next, a long narrow tube called a diagnostic catheter is advanced through the introducer over the guide wire, into the blood vessel. This catheter is then guided to the aorta and the guide wire is removed. Once the catheter is placed in the opening or ostium of one the coronary arteries, the doctor injects dye and takes an x-ray.

If a treatable blockage is noted, the first catheter is exchanged for a guiding catheter. Once the guiding catheter is in place, a guide wire is advanced across the blockage, then a balloon catheter is advanced to the blockage site. The balloon is inflated for a few seconds to compress the blockage against the artery wall. Then the balloon is deflated.

The doctor may repeat this a few times, each time pumping up the balloon a little more to widen the passage for the blood to flow through. This treatment may be repeated at each blocked site in the coronary arteries. A device called a stent may be placed within the coronary artery to keep the vessel open. Once the compression has been performed, contrast media is injected and an x-ray is taken to check for any change in the arteries. Following this, the catheter is removed and the procedure is completed.

The growing embryo requires nutrition and oxygen, and a disposal system for the waste products of its own metabolism. All of this is accomplished by the placenta, which allows the growing embryo to eat and breathe while in the mother’s uterus.

Following implantation of the fertilized egg into the uterine lining, the outer layer of the embryo develops spaces called lacunae. The lacunae filled up with blood from the mother’s uterine lining. Small projections from the embryo’s chorionic layer reached out into the uterine lining. The chorionic layer is one of the membranes that surround the embryo and help it implant. Blood vessels begin to form beneath this chorionic layer.

Around day 21, the embryo’s bloodstream and the mother’s bloodstream are in such close contact that nutrients and oxygen can cross from mother to embryo. The two bloodstreams are separated by a thin collection of tissues in the placenta called the blood barrier. This barrier permits small particles like nutrients and oxygen to pass from the mother to the embryo and allows waste products to pass from the embryo back to the mother.

The blood barrier also prevents many large or potentially harmful particles from entering the embryo’s bloodstream. The red blood cells do not cross from the mother’s bloodstream to the embryo’s bloodstream.

It’s important to keep the two bloodstreams separate since the blood type of the mother and embryo could be different. If the mother’s blood type is positive, and her embryo’s blood type is negative, then the mother’s blood cells would treat the embryo as an invading foreign organism, and try to destroy it.

The placenta and its blood barrier are important for supplying the growing embryo with nutrition and oxygen, removing its waste products, and preventing harmful substances from getting into the embryo’s bloodstream.

A woman is born with all of the egg cells she will release throughout her lifetime. Starting at about age 12 through menopause, a woman’s reproductive cycle releases an egg about once a month.

Hormonal messages from the brain instruct the ovaries to develop several follicles in which a single dominant follicle in one of the ovaries will release an egg for fertilization. During this time, other hormones instruct the uterine lining to thicken in preparation for nourishing a fertilized egg.

There are several hormones that regulate the reproductive cycle. Follicle stimulating hormone (FSH) stimulates preparation of the egg for fertilization by instructing a follicle to begin dividing it’s genetic material (chromosomes).

The follicle then releases estrogen, the hormone that prepares the lining of the uterus to receive a fertilized egg. Increased levels of estrogen in the bloodstream cause a small structure in the brain, the pituitary gland, to stop releasing the hormone FSH, and to start releasing luteinizing hormone (LH).

LH causes the follicle to enlarge rapidly and to release its egg in a process known as ovulation. Once the egg is out of the follicle, the follicle begins secreting the hormone progesterone, which also helps to prepare the uterine lining for the fertilized egg. The remaining cells of the follicle shrink into a hormone producing mass of cells called a corpus luteum.

The egg is swept into the fallopian tube by its waving structures called fimbriae. Fertilization of the egg usually occurs in the fallopian tube. From there, it is transported to the uterus and implants itself in the uterine wall, where it is nourished by the uterine lining. In the ovary, the corpus luteum produces progesterone so that the egg can develop into a fetus.

If the egg is not fertilized within 24 hours after its release from the ovary, it stops developing and dissolves before reaching the uterus. The absence of a fertilized egg causes the body to stop releasing the hormones that prepare the uterus for implantation. In response, the uterus sheds its lining over a period of four to five days in a process known as menstruation.

Blood carries various substances that must be brought to one part of the body or another. Red blood cells are an important element of blood. Their job is to transport oxygen to the body’s tissues in exchange for carbon dioxide, which is carried to and eliminated by the lungs.

Red blood cells are formed in the red bone marrow of bones. Stem cells in the red bone marrow called hemocytoblasts give rise to all of the formed elements in blood. If a hemocytoblast commits to becoming a cell called a proerythroblast, it will develop into a new red blood cell.

The formation of a red blood cell from hemocytoblast takes about 2 days. The body makes about two million red blood cells every second.

Blood is made up of both cellular and liquid components. If a sample of blood is spun in a centrifuge, the formed elements and fluid matrix of blood can be separated from each other. Blood consists of 45% red blood cells, less than 1% white blood cells and platelets, and 55% plasma.

The skeletal muscles are under voluntary (conscious) control most of the time. However, skeletal muscle movement can also by induced by involuntary reflexes.

Reflexes are involuntary reactions to a stimulus such as the burning of the hand. As soon as a hot substance contacts the hand, pain receptors in the skin send a signal to the spinal cord. In turn, the spinal cord sends a signal back to the arm muscles that instruct the hand to pull away. The arm flexed as it withdrew, which is known as a flexor (withdrawal) reflex. There are many other reflexes that protect the body as well.

If the body did not have the reflexes to withdraw quickly from a painful stimulus, we would be at risk for serious injury.

The eye is the organ of sight and is shaped as a slightly irregular hollow sphere. Various structures in the eye enable it to translate light into recognizable images. Among these are the cornea, the lens, and the retina.

Light first passes through the cornea, a clear dome-like structure covering the iris, or colored part, of the eye. The cornea bends, or refracts, the light onto the lens. The light is then refracted a second time while passing through the lens, finally focusing on the retina. The retina is the light sensitive part of the eye. Impulses travel down the optic nerve to the occipital lobe of the brain, which then interprets the image in the correct perspective.

The shape of the eye is very important in keeping the things we see in focus. If the shape of the eye changes, it affects a person’s vision.

Normally, light is precisely focused onto the retina at a location called the focal point. A nearsighted eye is longer from front to back than a normal eye causing light to be focused in front of the retina instead of directly onto it. This makes it difficult to see objects that are far away. Glasses with concave lenses are used to correct nearsightedness. The concave lens focuses light back onto the focal point of the retina.

Farsightedness occurs when the length of the eye is too short. Light is focused at a point behind the retina, making it difficult to see objects that are up close. A convex lens is used to correct farsightedness because it directs the focal point back onto the retina.

A baby's sex is determined at the time of conception. When a baby is conceived, the X or Y chromosome carried by the sperm cell fuses with the X chromosome in the egg cell. The chromosome combination determines whether the baby will be female or male. An XX combination means the baby will be a girl and XY means it will be a boy.

Even though gender is determined at conception, the fetus doesn’t develop its external sexual organs until the fourth month of pregnancy. At seven weeks after conception, the front of the fetus appears to be sexually indifferent, looking neither like a female or a male.

Over the next five weeks, the fetus begins producing hormones that cause its sex organs to grow into either female or male organs. This process is called sexual differentiation. If the fetus is female, it will produce hormones called estrogens. If the fetus is a male, it will produce hormones called androgens.

Hormones will instruct a common structure called the genital tubercle to either form the clitoris in the female or the penis in the male. The clitoris and penis are called sexual analogs because they originate from the same structure.

A baby's skeleton begins as fragile membranes and cartilage. As the fetus develops, the membranes and cartilage turn into bone in a process called ossification.

During the third month of development, the membranes on the side and back of the fetus’ skull start to ossify. Bone tissue slowly grows over the area where the membranes once existed. Eventually, these bone plates will grow together forming the cranial cavity which protects the brain.

Close to birth, the bones of the skull still have gaps between them called fontanelles. The fontanelles allow room for the baby's brain to grow and enable the head to be compressed during delivery.

Most of the bones of the skeleton start off as cartilage, such as the arms, legs, ribs, fingers, and backbone. From the second month until the end of the third month, the cartilage in the middle of the bones begins to ossify outward. Bones continue to grow in this manner until adulthood, allowing them to increase in their length and width.

Skeletal muscle is well-organized body tissue, composed in a complex array of smaller and smaller structures. Each skeletal muscle is composed of many units called muscle fascicles. The fascicles are bound together by a type of connective tissue called fascia.

Fascicles are composed of smaller organizational units called muscle fibers.

Smaller strands called myofibrils organize muscle fibers. The myofibrils move as skeletal muscle contracts. It is the interaction of the myofibrils as they slide and pull along side each other that gives skeletal muscle its functional ability to do work and move things.

Putting it all back together, myofibrils compose muscle fibers, muscle fibers make-up muscle fascicles, and muscle fascicles are bound together by fascia to compose skeletal muscle.

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The lungs are the primary respiratory organs. They act as filters for the air the body breathes in and normally are a healthy pink color.

Filtering smoke from the air breathed in can do damage to the lung tissue as seen in a smoker’s lung. Over time, carbon molecules from inhaled smoke deposit in the lung tissue, giving it a blackened appearance.

Smoking can eventually lead to the formation of tumors and other serious lung diseases. Smoking has also been linked to diseases that affect the cardiovascular system, such as atherosclerosis, which can lead to a heart attack or stroke.

Snoring affects many of people during their sleep when the airway become partially blocked, forcing the lungs to inhale harder to compensate for the lack of air entering the body. The snoring sound results from the vibration the soft palate and the uvula.

Several factors are thought to cause snoring, including poor muscle tone, too much alcohol, heavy smoking, colds or allergies, obesity, and obstruction by enlarged adenoids and tonsils.

Usually, snoring is not an indication of an underlying disorder. However, people who snore and have quiet periods lasting more than 10 seconds may have some degree of sleep apnea.

The key male reproductive organs include the testes, epididymis, urethra, vas deferens, prostate gland, seminal vesicle, and penis.

The testes are composed of coiled structures called seminiferous tubules, which are the sites of sperm production. The structure on top of the seminiferous tubules in the testes is the epididymis. The sperm migrate from of the seminiferous tubules to the epididymis. Within the epididymis, the sperm mature while they are stored in this structure.

The ejaculation process begins as the penis fills with blood and becomes erect. With sufficient stimulation, mature sperm travel from the epididymis through the vas deferens, a muscular tube, which propels sperm forward through smooth muscle contractions. The sperm arrive first at the ampulla, where secretions from the seminal vesicle are added.

From the ampulla, seminal fluid is propelled forward through the ejaculatory ducts toward the urethra, passing first by the prostate gland, where a milky fluid is added to form semen. Finally, the semen is ejaculated through the far end of the urethra.

The tongue has about 10,000 taste buds. The taste buds are linked to the brain by nerve fibers. Food particles are detected by the taste buds, which send nerve signals to the brain.

Certain areas of the tongue are more sensitive to certain tastes, like bitter, sour, sweet, or salty. Often, taste sensations are a mixture of these qualities.

• It helps keep the baby warm
• Provides lubrication preventing the baby’s body parts from growing together
• Enables the baby to move easily to exercise muscles and strengthen bones before being born
• Acts like a liquid shock absorber for the baby by distributing any force that may push on the mother’s uterus

Ultrasound is a useful procedure for monitoring the baby’s development in the uterus. Ultrasound uses inaudible sound waves to produce a two-dimensional image of the baby while inside the mother’s uterus. The sound waves bounce off solid structures in the body and are transformed into an image on a monitor screen.

Solid structures, such as bones and muscles, reflect sound waves and appear as light gray or white. Soft or hollow areas, like the chambers of the heart, don’t reflect sound waves and appear dark or black.

An ultrasound can supply vital information about a mother’s pregnancy and her baby's health. Even though there are no known risks for ultrasound at present, it is highly recommended that pregnant women consult their physician before undergoing this procedure.

The urinary system has four main components: the kidneys, ureters, urinary bladder, and urethra. Urine, a liquid waste product, is formed in the kidneys. From there it moves through the ureters and into the bladder, where it is stored. When the bladder gets full, urine is emptied from the body through the urethra in a process called urination.

The creation of urine is a complex process. The kidneys filter waste from the blood that passes through them, and reabsorb substances that the body requires, even though those requirements may change from moment to moment.

Each of the kidneys is composed of approximately one million subunits called nephrons. Each nephron consists of a microscopic ball of blood vessels called a glomerulus, which is connected to a twisting length of tube called the renal tubule. Because the blood vessels in the glomeruli are porous, they act as filters, removing most of the water, salt, and waste from the blood that passes through them.

As filters, the glomeruli have physical properties that prevent large cells, like red blood cells, from passing into the renal tubules. On the other hand, smaller particles, like sugar and salt, can pass easily through the glomerulus. Within the renal tubules, waste products are passed into the urine. Simultaneously, substances the body needs, such as water and salt, are reabsorbed back into the bloodstream.

The path of urine formation, reabsorption, and excretion begins at the glomerulus, continues through the renal tubules, and proceeds through a ureter into the bladder. The unique, expandable cells in the wall of the bladder stretch and become thinner as it fills. Finally, urine is excreted through the urethra.

Allergens, like pollen, are nothing more than foreign plant antigens. When they get released into the air, you can see and hear the result.When allergens first encounter nasal tissue, sneezing is triggered. This is part of the body’s immune defense.

Pollen allergens then encounter the plasma cells in the nose, which respond by producing antibodies. These antibodies attach to mast cells, which are white blood cells containing the chemical histamine. As more antibodies are produced, they cause the mast cells to release histamine, which produces allergy symptoms such as a stuffy and runny nose, sneezing, and watery eyes. These help to remove the invading pollen.

Medications called antihistamines can be used to help relieve severe allergy symptoms.

A plaque is an abnormal cluster of protein fragments. Such clusters can be found between nerve cells in the brain of someone with Alzheimer. A microscope will also show damaged nerve cells. In them are tangles called neurofibrillary tangles. These consist of twisted strands of a different protein.

Together, the plaque and tangles cause structural and chemical problems. It appears that Alzheimer disease causes parts of the brain that normally work together to disconnect.

While playing basketball, this player sprained his ankle. Probably not a good idea to play without shoes! Symptoms of a sprain include swelling and discoloration near the affected area.

A sprain occurs when a ligament is stretched or torn. Ligaments, such as those of the ankle, are bands or sheets of regular, tough fibrous tissue that connect bones together.

Here is the normal state of the ankle and it’s ligaments. Depending on the severity of the injury, the ligaments may be stretched, as in a Type One ankle sprain. Slightly torn, classified as a Type Two sprain, or completely torn, making it a Type Three sprain.

Treatment for a Type One sprain should include Rest, Ice, Compression and Immobilization, and Elevation of the affected area. This is easy to remember if you think of the acronym RICE.

If you suspect a ligament is torn or completely severed, see your medical care professional for treatment.

A change in the heart’s normal electrical conduction system can result in an arrhythmia, or irregular heartbeat.

An arrhythmia can be an abnormally slow heartbeat, or an abnormally fast heartbeat. In some cases, it can be fatal.

Atherosclerosis is a disease in which fatty material is deposited on the wall of an artery. Normally, the walls of an artery are smooth, allowing blood to flow unimpeded. However, if damage occurs to its inner lining, fat, cholesterol, platelets, and other substances may accumulate at a damaged section of the arterial wall.

Eventually, the tissue builds up and a plaque is formed, narrowing the lumen of the artery. Where the narrowing is severe, there is a risk that the vessel can become blocked completely if a thrombus forms in the diseased segment.

Athetosis is a condition marked by constant writhing movements. It's often caused by injury to basal ganglia. These are structures found deep within the brain near its base. They help coordinate muscle movement. The basal ganglia are highlighted here.

Balloon angioplasty is a procedure used to open narrowed or blocked arteries. It uses a balloon attached to a catheter that's inserted into an artery. At the place where deposits of plaque have closed off or narrowed the channel for blood flow, the balloon is inflated.

If the blockage is not major, it may be possible to correct the problem by inflating the balloon several times. This will compact the plaque against the wall, widening the passage and letting blood flow through.

It's common then, for a tubular device called a stent to be put into the artery. It will act like a scaffold inside the artery and keep the blood vessel open.

When the bladder fills with urine, sensory nerves send impulses to the brain telling it the bladder is full. The nerves connect with other nerves in the spinal cord to relay this information. In turn, the brain sends impulses back to the bladder instructing it to empty its contents.

The nervous system enables a person to blink to prevent harmful substances, like smoke from getting in the eyes. During the normal course of a day, a person blinks an average of 15 times a minute to keep the eyes healthy.

The lacrimal, or tear, gland provides lubricating fluid for the eyes. The eyelid moves fluid from the lacrimal gland and across the eye. Blinking also provides the eyes with protection from foreign objects.

When the eye becomes irritated, the lacrimal gland produces extra tears to wash out impurities. Excess fluid drains through the tear ducts and canal into the nasal cavity. An abundance of tears draining through the nasal cavity may cause the nose to run and a person to sniffle.

Ouch!

Here's how platelets form clots. This small artery has a cut. Blood flowing past the cut includes red blood cells that carry oxygen, platelets that come from white blood cell fragments, and clotting factors that help blood clot. When a blood vessel is damaged, blood cells and plasma ooze into surrounding tissue. Platelets immediately stick to the edges of the cut and release chemicals that attract more platelets. Eventually, a platelet plug is formed, and the outside bleeding stops.

On the inside, clotting factors cause a cascade of activity that includes strands of blood-borne material called fibrin sticking together to seal the inside of the wound. Eventually, the blood vessel heals, and several days later, the blood clot dissolves.

As the heart pumps, arteries, shown here in red, carry oxygen-rich blood away from the heart toward the body's tissues and vital organs. These include the brain, liver, kidneys, stomach, and muscles -- even the heart muscle.

At the same time, veins, shown here in blue, carry oxygen-poor blood from the tissues back to the heart. From there, it passes to the lungs to get more oxygen to take back to the heart so the cycle can repeat.

The force of blood on artery walls is called blood pressure. Normal pressure is important for the proper flow of blood from the heart to the body's organs and tissues. Each heart beat forces blood to the rest of the body. Near the heart, pressure is higher, and away from it lower.

Blood pressure depends on many things, including how much blood the heart is pumping and the diameter of the arteries the blood is moving through. Generally, the more blood that's pumped and the narrower the artery the higher the pressure is. Blood pressure is measured both as the heart contracts, which is called systole, and as it relaxes, which is called diastole. Systolic blood pressure is measured when the heart ventricles contract. Diastolic blood pressure is measured when the heart ventricles relax.

A systolic pressure of 115 millimeters of mercury is considered normal, as is a diastolic pressure of 70. Commonly, this pressure would be stated as 115 over 70.Stressful situations can temporarily cause blood pressure to rise. If a person has a consistent blood pressure reading of 140 over 90, he would be evaluated for high blood pressure.

Left untreated, high blood pressure can damage important organs, such as the brain and kidneys, as well as lead to a stroke.

If a fracture is severe, a bone graft may be used to help speed the healing process. Here, a metal plate is also used and fixated with screws. The plate and screws will be removed after the bone has healed.

The brain is composed of more than a thousand billion neurons. Specific groups of them, working in concert, provide us with the capacity to reason, to experience feelings, and to understand the world. They also give us the capacity to remember numerous pieces of information.

There are three major components of the brain. The cerebrum is the largest component, extending across the top of the head down to ear level. The cerebellum is smaller than the cerebrum and located underneath it, behind the ears toward the back of the head. The brain stem is the smallest and is located under the cerebellum, extending downward and back toward the neck.

The cerebral cortex is the outside portion of the cerebrum, also called the “gray matter”. It generates the most complex intellectual thoughts and controls body movement. The cerebrum is divided into left and right sides, which communicate with each other through a thin stalk of nerve fibers. The grooves and folds increase the cerebrum’s surface area, allowing us to have a tremendous amount of gray matter inside of the skull.

The left side of the brain controls the muscles on the right side of the body and vice versa. Here, the left side of the brain is highlighted to show the control over right arm and leg movement, and the right side of the brain is highlighted to show the control over left arm and leg movement.

Voluntary body movements are controlled by a region of the frontal lobe. The frontal lobe is also where we shape emotional reactions and expressions

There are two parietal lobes, one on each side of the brain. The parietal lobes are located behind the frontal lobe towards the back of the head and above the ears. The taste center is located in the parietal lobes.

All sounds are processed in the temporal lobe. They are also important for learning, memory, and emotion. The occipital lobe is located at the back of the head behind the parietal and temporal lobes.

The occipital lobe analyzes visual information from the retina and then processes that information. If the occipital lobe becomes damaged, a person could become blind, even if his or her eyes continue to function normally

 The cerebellum is located at the back of the head underneath the occipital and temporal lobes. The cerebellum creates automatic programs so we can make complex movements without thinking.

The brain stem is located underneath the temporal lobes and extended down to the spinal cord. It is critical for survival because it connects the brain with the spinal cord. The top portion of the brainstem is called the midbrain. The midbrain is a small portion of the brain stem located at the top of the brain stem. Just below the midbrain is the pons, and below the pons is the medulla. The medulla is the part of the brain stem closest to the spinal cord. The medulla, with its critical functions, lies deep within the head, where it is well-protected from injuries by an extra-thick section of overlying skull. When we are asleep or unconscious, our heart rate, breathing and blood pressure continue to function because they’re regulated by the medulla.

And that concludes a general overview of the components of the brain.

In a breast lift, excess skin and breast tissue are removed. Several different incision patterns can be used to reposition the areola and nipple, and to remove excess tissue.

In this example, an arch-like incision above the nipple shows where excess skin and breast tissue will be removed to accommodate the raised nipple. After the nipple is raised, stitches follow the circumference of the areola.

To complete the reshaping, another incision is made vertically from the bottom of the areola to the breast's natural lower crease.

The two lungs are the primary organs of the respiratory system. They sit to the left and right of the heart, within a space called the thoracic cavity. The cavity is protected by the rib cage. A sheet of muscle called the diaphragm serves other parts of the respiratory system, such as the trachea, or windpipe, and bronchi, conduct air to the lungs. While the pleural membranes, and the pleural fluid, allow the lungs to move smoothly within the cavity.

The process of breathing, or respiration, is divided into two distinct phases. The first phase is called inspiration, or inhaling. When the lungs inhale, the diaphragm contracts and pulls downward. At the same time, the muscles between the ribs contract and pull upward. This increases the size of the thoracic cavity and decreases the pressure inside. As a result, air rushes in and fills the lungs.

The second phase is called expiration, or exhaling. When the lungs exhale, the diaphragm relaxes, and the volume of the thoracic cavity decreases, while the pressure within it increases. As a result, the lungs contract and air is forced out.

Bunions are usually caused by pressure put on the feet by narrow toed, high-heeled shoes. The shoes press on the big toe and push it toward the second toe. Over time, the condition can become painful. The pain comes from the growth of extra bone where the base of the big toe meets the foot.

Malignant tumors, as seen developing on the right vocal cord, are typically caused by tobacco use.

A network of specialized muscle cells is found in the heart's walls. These muscle cells send signals to the rest of the heart muscle causing a contraction. This group of muscle cells is called the cardiac conduction system.

The main parts of the system are the SA node, AV node, bundle of HIS, bundle branches, and Purkinje fibers.

Let's follow a signal through the contraction process. The SA node starts the sequence by causing the atrial muscles to contract. That's why doctors sometimes call it the anatomical pacemaker.

Next, the signal travels to the AV node, through the bundle of HIS, down the bundle branches, and through the Purkinje fibers, causing the ventricles to contract.

This signal creates an electrical current that can be seen on a graph called an electrocardiogram, or EKG. Doctors use an EKG to see how well the cardiac conduction system works. Any changes on the EKG can mean serious problems.

Cardiomyopathy refers to heart muscle disease. Here, a diseased section of wall that separates the heart chambers that pump blood out partly blocks the flow through one of them. The condition is called hypertrophic obstructive cardiomyopathy.

To treat it, a catheter is inserted into the heart. It's used to apply concentrated alcohol that shrinks the diseased section. Now the heart can function normally.

The heart is a powerful automatic pump. It's the part of the cardiovascular system we think of most when we think about good health. But healthy blood and blood vessels are also vital for staying well.

 The average adult has between 5 and 6 liters of blood or blood volume. Blood carries oxygen and nutrients to all the living cells in the body. It also carries waste products to systems that eliminate them.

Half the blood consists of a watery, protein-laden fluid called plasma. A little less than half is composed of red and white blood cells and other solid elements called platelets. Platelets cause the blood to coagulate wherever an injury to a blood vessel occurs.

Most cataracts are age related. They show up as a cloudy area in the lens.

For the first 12 hours after conception, the fertilized egg remains a single cell. After 30 hours or so, it divides from one cell into two. Some 15 hours later, the two cells divide to become four. And at the end of 3 days, the fertilized egg cell has become a berry-like structure made up of 16 cells. This structure is called a morula, which is Latin for mulberry.

During the first 8 or 9 days after conception, the cells that will eventually form the embryo continue to divide. At the same time, the hollow structure in which they have arranged themselves, called a blastocyst, is slowly carried toward the uterus by tiny hair-like structures in the fallopian tube, called cilia.

The blastocyst, though only the size of a pinhead, is actually composed of hundreds of cells. During the critically important process of implantation, the blastocyst must attach itself to the lining of the uterus or the pregnancy will not survive.

If we take a closer look at the uterus, you can see that the blastocyst actually buries itself in the lining of the uterus, where it will be able to get nourishment from the mother’s blood supply.

Brain tissue is supplied with oxygen and nutrients by a network of cerebral arteries. If the wall of an artery becomes weak, a portion of it may balloon out and form an aneurysm.

A cerebral aneurysm may enlarge until it bursts. If it does, it will send blood throughout the spaces in or surrounding the brain.

Click and drag the slider bar to view cervical dilation.

If you have previously delivered a child, your cervix looks slot-shaped at 0 centimeters, not round.

During early labor your cervix dilates from 1 to 5 centimeters. Mild contractions, about 60 to 90 seconds in length, occur generally every 15 to 20 minutes.

You've entered the active stage of labor, which means your cervix is dilating from 5 to 8 centimeters, contractions have grown stronger and more insistent. They are occurring once every 3 minutes or so, and last for about 45 seconds.

Near the active phase of labor you arrive at the transition phase; your cervix dilates from 8 to 10 centimeters.

At 10 centimeters, your cervix is fully dilated and you are ready to start pushing with every contraction.

A cesarean section is a way to deliver a baby by cutting through the skin of the mother's abdomen. Although cesarean (C-sections) are relatively safe surgical procedures, they should only be performed in appropriate medical circumstances.

Some of the most common reasons for a cesarean are:

• If the baby is in a feet first (breech) position.
• If the baby is in a shoulder first (transverse) position.
• If the baby’s head is too large to fit through the birth canal.
• If labor is prolonged and the mother’s cervix will not dilate to 10 centimeters.
• If the mother has placenta previa, where the placenta is blocking the birth canal.
• If there are signs of fetal distress which is when the fetus is in danger because of decreased oxygen flow to the fetus.

Some common causes of fetal distress are:

• Compression of the umbilical cord.
• Compression of major blood vessels in the mother’s abdomen because of her birthing position.
• Maternal illness due to hypertension, anemia, or heart disease.

Like many surgical procedures, cesarean sections require anesthesia. Usually, the mother is given an epidural or a spinal block. Both of these will numb the lower body, but the mother will remain awake. If the baby has to be delivered quickly, as in an emergency, the mother may be given a general anesthetic, which will make her fall asleep. During the surgery, an incision is made in the lower abdomen followed by an incision made in the uterus. There is no pain associated with either of these incisions because of the anesthesia.

The doctor will open the uterus and the amniotic sac. Then the baby is carefully eased through the incision and out into the world. The procedure usually lasts about 20 minutes.

Afterward, the physician delivers the placenta and stitches up the incisions in the uterus and abdominal wall. Usually, the mother is allowed to leave the hospital within a few days, barring complications like wound infections. One concern that many women have is whether they’ll be able to have a normal delivery after having a cesarean. The answer depends on what the reasons were for having the c-section in the first place. If it was because of a one-time problem, like umbilical cord compression or breech position, then the mother may be able to have a normal birth.

Therefore, as long as the mother has had one or two previous cesarean deliveries with a low-transverse uterine incision, and there are no other indications for a cesarean, she is a candidate for vaginal birth after cesarean, also called VBAC.

Cesarean sections are safe, and can even save the lives of both mother and baby during emergency deliveries. Expectant mothers should be prepared for the possibility of having one. Keep in mind, in childbirth, it’s not only the delivery method that matters, but the end result: a healthy mother and baby.

The average adult has about 6 pounds of skin covering 18 square feet, making skin the body's largest organ. Let's look at how the skin is put together. Skin has three layers. The top layer is the epidermis. It protects the other layers from the outside environment. It contains cells that make keratin, which waterproofs and strengthens the skin. The epidermis also has cells with melanin, the dark pigment that gives skin its color. Other cells in the epidermis allow us to feel touch and provide immunity against invaders like bacteria and other germs.

The bottom layer is the hypodermis. It contains fat cells, or adipose tissue, that insulate the body and help conserve heat.Between the epidermis and hypodermis is the dermis. It contains cells that give skin strength, support, and flexibility.As we age, cells in the dermis lose their strength and flexibility, causing the skin to lose its youthful appearance.

The dermis has sensory receptors that allow the body to receive stimulation from the outside and feel pressure, pain, and temperature.A network of blood vessels provide the skin with nutrients, and remove waste products.

Sebaceous glands produce oil that keeps the skin from drying out. Oil from the sebaceous glands also helps to soften hair and to kill bacteria in the pores of the skin.

These glands cover the whole body, except for the palms of the hands and soles of the feet.

In this cut -away view you can see both the woman’s and the man’s reproductive organs during intercourse. Here are the penis, vagina, uterus, testicle and prostate gland.

During intercourse, sperm are released into the vagina near the cervix, which is the entrance to the uterus. Here you can see the sperm swimming through the uterus and up the fallopian tubes. From their profiles, you can see the sperm actually have 3 parts, a head, a middle section, and a tail. The tail propels the sperm, which is powered by energy cells stored in the middle section. The head of the sperm contains the man’s genetic material and an enzyme-filled acrosomal cap needed to help the sperm penetrate through the outer membrane of the egg.

Here’s the egg, moving through the fallopian tube. Within the fallopian tube, the egg is met by hundreds of sperm. Finally, one of the sperm succeeds in breaking through the egg’s outer membrane.

After penetrating the egg’s outer membrane, the sperm released its nucleus, which unites with the nucleus from the egg. Fertilization or conception occurs when the sperm fuses with the egg to form a fertilized egg. The fertilized egg is called a zygote.

Click a circle in the "Navigation" box to travel to a particular section of the female reproductive system. At each section, select the "Click here to play animation" to see an action occur that leads to the successful conception of a fertilized egg.

In this cut -away view you can see both the woman’s and the man’s reproductive organs during intercourse. Here are the penis, vagina, uterus, testicle and prostate gland.

During sexual intercourse, the sperm are released into the vagina near the cervix, which is the entrance to the uterus. Here you can see the sperm swimming through the uterus and up the fallopian tubes. From their profiles, you can see the sperm actually have 3 parts: a head, a middle section, and a tail, which propels them through the uterus.

If you take a closer look at the sperm's head, you’ll see that its covered with an enzyme "cap" that will help it break through the outer wall of the egg cell.

Also within the head are clumps of chromosomes. Chromosomes contain the genetic material, or genes, that are the hereditary blueprints that get passed on to the baby. If a sperm containing a Y chromosome fertilizes the egg, the baby will be a boy. If the lucky sperm contains an X Chromosome, then the baby will be a girl. In addition to a baby’s sex, the genes on the chromosomes determine thousands of other characteristics, including height, body shape, facial features and eye color, and may even influence characteristics like talent and aptitude.

Now let's see what's going on with the egg cell.

Here's the egg cell, moving through the fallopian tube. It can't swim by itself, so it gets moved along by the beating motion of tiny cilia that line the walls of the tube. Unless it gets fertilized, an egg can only survive for 12-24 hours after ovulation. Here you see the egg being met by the sperm. All of the sperm are trying to penetrate the egg.

Actually those sperm are the only remaining survivors of the millions of sperm that were released into the woman’s reproductive tract. The woman’s reproductive tract has an acidic lining and a host of cellular defense mechanisms, making it a hostile environment, and few sperm are strong enough to make it to the egg.

If you watch now you can see the process of fertilization beginning. When one of the sperm cells finally succeeds in breaking through the egg cell's outer membrane, you’ll see something remarkable happen. There it is! The egg cell is locking out other sperm cells from entering. This ensures that only one sperm cell fertilizes the egg cell. If more than one sperm cell was involved, the egg cell might not survive because it would have the wrong amount of genetic material.

Now, here's the final part of fertilization: the sperm cell releases its nucleus containing the father’s chromosomes and then after several hours it unites with the nucleus of the egg cell, which contains the mother’s chromosomes. And when the two nuclei fuse, their genetic material combines together to create a zygote, which is what a fertilized egg cell is called.

Millions of sperm are released during a single ejaculation. Their tails propel on their journey to encounter the single egg cell. Of the millions of sperm, only a few will survive to reach the egg and just one will penetrate the egg cell’s wall to combine it’s genetic material with that of the egg in the process called fertilization. If during the first week of cell division, the fertilized egg cell, or zygote, divides into 2 zygotes, identical twins will form. Each developing embryo contains the same genetic material as the other.

A blow to the head can cause the brain to move and hit the skull. Here, a punch in a boxing match shakes the brain from side to side. A concussion can cause major confusion due to brain distortion.

Injury or infection of the cornea, the eye's front window, can cause lasting problems with vision.

One option to treat the blocked coronary artery is a surgical procedure called coronary artery bypass grafting surgery. The procedure involves taking a section of blood vessel from elsewhere in the body, such as the leg, and grafting it onto a location on the heart, which allows blood to bypass the blocked portion of the coronary artery.

The procedure begins with the surgeon making a cut in the leg and removing a section of vein. Both ends of the vein are tied-off in the leg and cut is closed. The blood circulation in the leg is not compromised because the leg has many other veins, which can take over circulation in the area of the removed vein.

The surgeon will then divide the sternum, the bone that runs down the middle of the chest, exposing the chest cavity. The heart is then usually connected to a heart-lung machine, which takes over the work of the heart and lungs during the treatment. Once this is complete, the heart is stopped. The surgeon then locates the blocked coronary artery and attaches the section of vein taken from the leg to the aorta and to the coronary artery below the blocked segment of the artery. The surgeon may do as many bypasses on as many blocked coronary arteries as the patient needs.

Once each bypass graft is placed, it is checked for leaks. Following this, the heart is restarted. Once the heart is beating again, the surgeon will remove the leads to the heart-lung machine and sew the openings closed. Following this the chest is closed. The surgeon will leave pacemaker wires in the heart and bring them out to the skin to help control any heart rhythm problems the patient may have.

If we looked inside the chest cavity, we would see the lungs and pericardium, which is the fibrous covering of the heart. We could also see the heart beating. Here are the left and right coronary arteries. They supply blood to specific regions of the heart. If either is damaged or blocked, it could injure the heart wall.

In a healthy artery, red blood cells flow through unimpeded. But if the inner wall is damaged, cholesterol plaque can build up. This progressively narrows the space through which blood flows.

Eventually, the artery may become too narrow for clotted blood to pass through. If it does, the artery could become completely blocked.

That would cause a lack of oxygen, or ischemia, in the part of the heart the artery supplies. The result is a heart attack, known as a myocardial infarction.

Facial cosmetic surgery may include a forehead lift. In this procedure, a hairline incision is made, the forehead skin is pulled up and excess skin tissue is removed. In an eyelid lift, blepharoplasty, creases and wrinkles around the eyes can be minimized by removing excess fat and skin from the upper and lower eyelids. A facelift usually consists of an incision along or above the hairline and in front of the ears. Excess fat and skin is removed and facial muscles may be tightened.

Coughing is a sudden expulsion of air from the lungs through the epiglottis, cartilage located in the throat, at an amazingly fast speed. Compared to a tennis ball hit at 50 miles per hour, or a baseball at 85 miles per hour...coughing is faster, with an estimated speed of 100 miles per hour. With such a strong force of air, coughing is the body's mechanism for clearing the breathing passageways of unwanted irritants.

Let's take a look at the vocal cords prior to a cough.

In order for a cough to occur, several events need to take place in sequence. Let's use the unwanted irritant of water entering the windpipe, also known the trachea, to trigger the coughing reflex.

First, the vocal cords open widely allowing additional air to pass through into the lungs. Then the epiglottis closes off the windpipe, and simultaneously, the abdominal and rib muscles contract, increasing the pressure behind the epiglottis. With the increased pressure, the air is forcefully expelled, and creates a rushing sound as it moves very quickly past the vocal cords. The rushing air dislodges the irritant making it possible to breathe comfortably again.

Diabetes may affect the retina by causing the formation of whitish patches called exudates.

Other indications may include tiny enlargements of the blood vessels, resulting in microaneurysms and hemorrhages.

Food passes from the mouth through the esophagus to the stomach. The stomach churns the food and breaks it down further with hydrochloric acid and an enzyme called pepsin. The process of breaking food down in the stomach takes a few hours. From there, it goes to the duodenum, which the first part of the small intestine. Within the duodenum, digestive bile produced by the liver and stored in the gallbladder along with enzymes from the pancreas break it down more.

Enzymes are chemicals that speed up the digestion of specific types of food. For example, the enzyme trypsin breaks down the protein in steak, and lipase helps to break down fat. Humans don’t have enzymes to break down certain plant fibers, which is why they can’t be fully digested. The enzyme called lactase breaks down the sugar in milk. Sometimes, lactase is not produced by the body at all, or in insufficient amounts, making a person lactose intolerant. So, when a person who is lactose intolerant eats ice cream or yogurt, the digestive system gets bloated and expels gas.

Once everything is broken down, the small intestine absorbs the nutrients the body needs. From there the nutrients go into the bloodstream and to the liver, where poisons are removed. Undigested food and water continue through the small intestine and go into the large intestine, where water is reabsorbed. Then, at the end of the line, feces are eliminated through the rectum and anus.

DCA, or directional coronary atherectomy is a minimally invasive procedure to remove blockage from coronary arteries to improve blood flow to the heart muscle and ease pain.

First, a local anesthesia numbs the groin area. Then the doctor puts a needle into the femoral artery, the artery that runs down the leg. The doctor inserts a guide wire through the needle and then removes the needle. He replaces it with an introducer, a tubular instrument with two ports used to insert flexible devices such as a catheter into a blood vessel. Once the introducer is in place, the original guidewire is replaced by a finer wire. This new wire is used to insert a diagnostic catheter, a long flexible tube, into the artery and guide it to the heart. The doctor then removes the second wire.

With the catheter at the opening of one of the coronary arteries, the doctor injects dye and takes an X-ray. If it shows a treatable blockage, the doctor uses another guide wire to remove the first catheter and replace it with a guiding catheter. Then the wire that was used to do this is removed and replaced by a finer wire that is advanced across the blockage.

Another catheter designed for lesion cutting is also advanced across the blockage site. A low-pressure balloon attached next to the cutter, is inflated, exposing lesion material to the cutter.

A drive unit is turned on, causing the cutter to spin. The doctor advances a lever on the drive unit that in turn advances the cutter. The pieces of blockage it cuts away are stored in a section of the catheter called a nosecone until they are removed at the end of the procedure.

Rotating the catheter while inflating and deflating the balloon makes it possible to cut the blockage in any direction, leading to uniform debulking. A stent may also be placed. This is a latticed metal scaffold put inside the coronary artery to keep the vessel open.

After the procedure, the doctor injects dye and takes an X-ray to check for change in the arteries. Then the catheter is removed and the procedure is over.

After the membranes rupture and the water breaks, a woman may begin to experience the First Phase of labor, or Early labor. The average time of early labor is extremely variable, lasting anywhere from 2 to 6 hours. In rare cases, it can last up to 24 hours.

During this time, the pressure of repeated regular contractions causes the cervix, which had been closed when labor began, to open up to a diameter of 3 centimeters, and at the same time, become much thinner.

Various techniques can be used to help alleviate the discomfort a woman may experience during the first phase of labor such as back-rubs and breathing exercises.

For conception to take place, a mature egg cell, or ovum, must be at the right place at the right time. Conception takes place when a sperm penetrates the egg cell and fertilizes it, and the two cells combine to form a new life. Let's take a quick look at some of a woman’s key reproductive organs and see how they function during menstruation and ovulation, two processes that are critical in preparing her for conception.

Here are the uterus, ovaries, fallopian tubes, and vagina.

You can see a cut-away view of one of the ovaries on the right. The purple structures inside the ovary are immature egg cells, or oocytes. All of the 400,000 egg cells a woman will ever produce are already present in her ovaries when she is born, although the eggs are in an undeveloped form. The average age that girls begin to menstruate is 12 years old. Each menstrual cycle occurs approximately every twenty-eight days. During each cycle, hormonal messages from the brain cause the ovaries to develop a single mature egg cell for potential fertilization, even as other hormones instruct the uterine lining to thicken in preparation for nourishing the fertilized egg cell. As you may know, hormones are chemicals released into the blood stream by organs or glands. In general, their job is to regulate body functions by either stimulating or inhibiting other cells or organs. The ovaries are just one of the many organs in the body regulated by hormones. The cycle starts when a follicle grows within one of the ovaries. A follicle is composed of the developing egg cell and the support cells that surround and nourish it.

On day 1 of the cycle, a small structure in the brain, the pituitary gland, releases two hormones: FSH and LH, both of which cause the follicle to begin growing. Over the next 13 days, the growing follicle releases estrogen, a hormone that prepares the lining of the uterus to receive a fertilized egg cell. Meanwhile, the estrogen in the blood stream causes the brain to release a surge of LH. In response to the LH surge, the follicle enlarges rapidly. On day 14, it ruptures and releases the egg cell in a process known as ovulation. The ruptured follicle begins secreting the hormone progesterone, which also helps to prepare the uterine lining for a fertilized egg cell.

The large structure on the right is the entrance to the fallopian tube. The smaller, waving structures at its opening are called fimbriae. They're moving a lot because it's their job to sweep the egg cell into the fallopian tube's entrance and toward the uterus. Once the egg cell is within the fallopian tube, one of two things will happen to it: it will either be fertilized by a sperm cell, or fertilization will fail to take place.

If the egg cell is NOT fertilized within 12 to 24 hours after its release from the ovary, it will stop developing and will dissolve before reaching the uterus. The absence of a fertilized egg cell gradually causes a woman’s body to stop releasing the hormones that would otherwise prepare the uterus for the developing egg cell. In response, the uterus sheds its lining on days 24 through 28 during menstruation. If the egg cell DOES become fertilized by a sperm, it will be transported by tiny hair-like cells, called cilia, to the uterus. There, it lodges in the uterine wall in a process called implantation, and receives nourishment from the uterine lining. Meanwhile, back in the ovary, the remaining cells of the ruptured follicle produce progesterone so that the uterine lining will stay rich in blood vessels, and the fertilized egg cell will survive.

As you can see, the hormones, which control the reproductive system, maintain a delicate balance over the life cycle of the egg cell.

For conception to take place, a mature egg cell, or ovum, must be at the right place at the right time. Conception takes place when a sperm penetrates the egg cell and fertilizes it, and the two cells combine to form a new life. Let's take a quick look at some of a woman’s key reproductive organs and see how they function during menstruation and ovulation, two processes that are critical in preparing her for conception.

Here are the uterus, ovaries, fallopian tubes, and vagina.

You can see a cut-away view of one of the ovaries on the right. The purple structures inside the ovary are immature egg cells, or oocytes. All of the 30,000 egg cells a woman will ever produce are already present in her ovaries when she is born, although the eggs are in an undeveloped form. The average age that girls begin to menstruate is 12 years old. Each menstrual cycle occurs approximately every twenty-eight days. During each cycle, hormonal messages from the brain cause the ovaries to develop a single mature egg cell for potential fertilization, even as other hormones instruct the uterine lining to thicken in preparation for nourishing the fertilized egg cell. As you may know, hormones are chemicals released into the blood stream by organs or glands. In general, their job is to regulate body functions by either stimulating or inhibiting other cells or organs. The ovaries are just one of the many organs in the body regulated by hormones. The cycle starts when a follicle grows within one of the ovaries. A follicle is composed of the developing egg cell and the support cells that surround and nourish it.

On day 1 of the cycle, a small structure in the brain, the pituitary gland, releases two hormones: FSH and LH, both of which cause the follicle to begin growing.

Over the next 13 days, the growing follicle releases estrogen, a hormone that prepares the lining of the uterus to receive a fertilized egg cell. Meanwhile, the estrogen in the blood stream causes the brain to release a surge of LH. In response to the LH surge, the follicle enlarges rapidly.

On day 14, it ruptures and releases the egg cell in a process known as ovulation. The ruptured follicle begins secreting the hormone progesterone, which also helps to prepare the uterine lining for a fertilized egg cell. The large structure on the right is the entrance to the fallopian tube. The smaller, waving structures at its opening are called fimbriae. They're moving a lot because it's their job to sweep the egg cell into the fallopian tube's entrance and toward the uterus.

Once the egg cell is within the fallopian tube, one of two things will happen to it: it will either be fertilized by a sperm cell, or fertilization will fail to take place. If the egg cell is NOT fertilized within 12 to 24 hours after its release from the ovary, it will stop developing and will dissolve before reaching the uterus. The absence of a fertilized egg cell gradually causes a woman’s body to stop releasing the hormones that would otherwise prepare the uterus for the developing egg cell.

In response, the uterus sheds its lining on days 24 through 28 during menstruation. If the egg cell DOES become fertilized by a sperm, it will be transported by tiny hair-like cells called cilia to the uterus. There, it lodges in the uterine wall in a process called implantation, and receives nourishment from the uterine lining.

Meanwhile, back in the ovary, the remaining cells of the ruptured follicle produce progesterone so that the uterine lining will stay rich in blood vessels, and the fertilized egg cell will survive.

As you can see, the hormones, which control the reproductive system, maintain a delicate balance over the life cycle of the egg cell.

Click the waveform pull-down list to view various waveforms showing normal and pathological conditions of the heart.

The glands that make up the endocrine system produce chemical messengers called hormones that travel through the blood to other parts of the body.

Important endocrine glands include the pituitary, thyroid, parathyroid, thymus, and adrenal glands.

There are other glands that contain endocrine tissue and secrete hormones, including the pancreas, ovaries, and testes.

The endocrine and nervous systems work closely together. The brain sends instructions to the endocrine system. In return, it gets constant feedback from the glands.

The two systems together are called the neuro endocrine system.

The hypothalamus is the master switchboard. It's the part of the brain that controls the endocrine system. That pea-sized structure hanging below it is the pituitary gland. It's called the master gland because it regulates the activity of the glands.

The hypothalamus sends either hormonal or electrical messages to the pituitary gland. In turn, it releases hormones that carry signals to other glands.

The system maintains its own balance. When the hypothalamus detects the rising level of hormones from a target organ, It sends a message to the pituitary to stop releasing certain hormones. When the pituitary stops, it causes the target organ to stop producing its hormones.

The constant adjustment of hormone levels lets the body function normally.

This process is called homeostasis.

The prostate is a male gland located underneath the bladder and is about the size of a chestnut. In this cut section, you can see that part of the urethra is encased within the prostate gland. As a man ages, the prostate typically enlarges in size in a process called BPH, which means that the gland gets larger without becoming cancerous. The enlarged prostate crowds its anatomical neighbors, particularly the urethra, causing it to narrow.

The narrowed urethra results in several of the symptoms of BPH. Symptoms may include a slowed or delayed start in urination, the need to urinate frequently during the night, difficulty in emptying the bladder, a strong, sudden urge to urinate, and incontinence. Less than half of all men with BPH have symptoms of the disease, or their symptoms are minor and do not restrict their life style. BPH is a normal physiological process of aging.

Treatment options are available and are based on the severity of the symptoms, the extent to which they affect lifestyle, and the presence of other medical conditions. Men with BPH should consult with their physician yearly to monitor the progression of the symptoms and decide the best course of treatment as needed.

Imagine you're getting ready to race. The stress you feel makes your brain signal the adrenal glands to produce epinephrine or "adrenaline". Adrenaline increases your heart rate. As a result, more oxygen gets to your muscles. That makes your body ready to react. In a longer-term response to stress, the glands secrete cortisol. Cortisol promotes the release of energy, making you ready to race.

Weight lifting is a form of anaerobic exercise. It is very demanding, requiring a great deal of energy, which quickly depletes the body’s oxygen reserves. Sprinting and push-ups are other examples of anaerobic activities. They each create a situation called oxygen debt, which requires us to breathe deeply and rapidly in order to restore a proper oxygen level to the muscle cells.

Let’s take a look at this woman exercising her right biceps muscle. Repetitions of this exercise cause her biceps muscle fibers to increase in size. As she works out, if no more oxygen is available, her muscles convert a starch, called glycogen, into energy. This conversion process creates a waste product called lactic acid, which can be partly responsible for her muscle soreness the next day.

Conversely, jogging is a form of aerobic exercise. Exercising over a long duration requires a steady level of energy for the body. If properly conditioned, the body will be able to supply adequate oxygen to meet its energy requirements during aerobic exercise and much less lactic acid will be formed in the muscles.

Ouch!!! Gotcha!

Pain, although often uncomfortable, is a protective mechanism that alerts us to potential or actual harm to the body's tissues.

Here, the peripheral nervous system sent a pain message to the brain that a bee sting occurred on the nose.

Let's take a look at an instant replay to see how this communication works.

The pain receptors in the skin detect tissue damage from the bee sting. Then, the peripheral nerves send a pain signal to the brain. The brain analyzes the pain signal.

Ouch!!!

In turn, the brain delivers a message back to the muscles of the arm to react.

Hasta la beesta, babee!

As you can tell, it's a very effective system.

Click and drag a slider bar underneath an image window to see the process of embryonic and fetal development.

During the fifth month, a baby’s outer ears are almost fully developed. Ear formation starts from a few small lumps during the second month. Let’s go and take a look.

Here we see a baby during the fifth week of development. Those small bulges are called branchial arches. During month three, the branchial arches formed the lower face and neck. The ears developed from them, too, between those two branchial arches. Let’s take a closer look. In the fifth week, they were smooth. But a week after that, tiny bumps called auricular hillocks formed on each branchial arch. Now here’s the fascinating part. Watch what happens from the sixth week until the end of the fifth month...As you can see, the auricular hillocks grew and joined together to form the baby’s outer ears.

During the fifth month, the inner and middle part of the baby’s ears are also developing, but they won’t be completely finished until birth. And here’s what they’ll look like at that time.

Twins are rare and special, occurring in about 2% of all pregnancies. Of that number, 30% are identical twins. The other 70% are non-identical, or fraternal twins.

This animation will show you the differences between the development of a single baby, identical twins, and fraternal twins.

Starting with the single baby, let’s go back to the beginning, when fertilization occurs. Here you see that the egg cell is fertilized by a single sperm cell to form a zygote. Over the next few days, the fertilized egg cell divides over and over to form a structure composed of hundreds of cells called a blastocyst.

During the first week after fertilization, we can look inside the blastocyst and see the mass of cells that will form the embryo. The blastocyst will continue traveling toward the uterus where it will implant in the uterine lining, and grow into a single baby.

Now let’s watch the development of identical twins. Identical twins start out from a single fertilized egg cell, or zygote, which is why they’re also called monozygotic twins. Like the single baby we just saw, the egg cell is fertilized by a single sperm cell.

Unlike the single baby, this fertilized egg cell will split into two separate embryos, and grow into identical twins. This remarkable event takes place during the first week after fertilization, and can happen at several different times: at the two cell stage on day 2 at the early blastocyst stage on day 4 or in the late blastocyst stage on day 6.

The stage at which the egg cell splits determines how the twins will implant in the uterine lining, and whether or not they share an amnion, chorion, and placenta. Basically, the earlier the splitting occurs, the more independently the twins will develop in the uterus. So, a pair of identical twins that split during the two-cell stage will each develop its own amnion, chorion, and placenta.

Twins that split during the late blastocyst stage will share an amnion, chorion, and amniotic sac.

A common misconception about the conception of identical twins is that the trait for having them is passed on to future generations through the mother’s genes. But the truth is science doesn’t know the reason why identical twins occur. At this time, we can just say that they’re examples of a nine-month double miracle.

Now let’s take a look at the second type of twins. Non-identical, or fraternal, twins develop from two fertilized egg cells, or zygotes. Which is why they’re also called dizygotic twins. Unlike identical twins, however, fraternal twins are definitely influenced by the mother’s genes. Here’s why:

When the mother of fraternal twins ovulates, sometimes her ovaries release two egg cells for fertilization. Typically, only one egg cell is released during ovulation.

During conception, both of these egg cells become fertilized by two different sperm cells, which is why fraternal twins don’t look exactly alike. Sometimes they’re not even the same sex.

Here in the uterus, you can see that the twin embryos develop separately each having his or her own chorion, amnion, and placenta.

Air enters the body through the mouth or nose and quickly moves to the pharynx, or throat. From there, it passes through the larynx, or voice box, and enters the trachea.

The trachea is a strong tube that contains rings of cartilage that prevent it from collapsing.

Within the lungs, the trachea branches into a left and right bronchus. These further divide into smaller and smaller branches called bronchioles.

The smallest bronchioles end in tiny air sacs. These are called alveoli. They inflate when a person inhales and deflate when a person exhales.

During gas exchange oxygen moves from the lungs to the bloodstream. At the same time carbon dioxide passes from the blood to the lungs. This happens in the lungs between the alveoli and a network of tiny blood vessels called capillaries, which are located in the walls of the alveoli.

Here you see red blood cells traveling through the capillaries. The walls of the alveoli share a membrane with the capillaries. That's how close they are.

This lets oxygen and carbon dioxide diffuse, or move freely, between the respiratory system and the bloodstream.

Oxygen molecules attach to red blood cells, which travel back to the heart. At the same time, the carbon dioxide molecules in the alveoli are blown out of the body the next time a person exhales.

Gas exchange allows the body to replenish the oxygen and eliminate the carbon dioxide. Doing both is necessary for survival.

Glaucoma is unhealthy pressure inside the eye. Untreated, it can damage the optic nerve, causing vision loss and blindness.

Gout is a common, painful form of arthritis. It causes swollen, red, hot and stiff joints. Gout is caused by increased production of uric acid.

Uric acid crystals travel and accumulate in the joints, especially in the feet and legs, causing great pain and swelling.

You are more likely to get gout if you are a man, overweight, drink alcohol, eat too many foods rich in purines, or have a family member with gout.

The ear is divided into three regions: the outer ear, the middle ear and the inner ear. When sound waves enter the ear canal, they cause the eardrum to vibrate. The vibration moves the three bones in the middle ear, called the ossicles. The ossicles are also known as the hammer, anvil, and stirrup. These tiny bones transfer and amplify sound waves to the oval window, which is located behind the stirrup.

When the oval window vibrates, it moves fluid across a membrane inside the cochlea. The fluid causes the membrane to move.

Specialized hair cells translate this movement into nerve impulses, which are sent to the brain through the vestibulocochlear nerve.

The brain interprets the impulses as sound.

Sound waves entering the ear travel through the external auditory canal before striking the eardrum and causing it to vibrate.

The eardrum is connected to the malleus, one of three small bones of the middle ear. Also called the hammer, it transmits sound vibrations to the incus, which passes them to the stapes. The stapes pushes in and out against a structure called the oval window. This action is passed onto the cochlea, a fluid-filled snail-like structure that contains the organ of Corti, the organ for hearing. It consists of tiny hair cells that line the cochlea. These cells translate vibrations into electrical impulses that are carried to the brain by sensory nerves.

In this cut-view, you can see the organ of Corti with its four rows of hair cells. There is an inner row on the left and three outer rows on the right.

Let's watch this process in action.First, the stapes rocks against the oval window. This transmits waves of sound through the cochlear fluid, sending the organ of Corti into motion.

Fibers near the upper end of the cochlea resonate to lower frequency sound. Those near the oval window respond to higher frequencies.

Heart bypass surgery creates a new route, called a bypass, for blood and oxygen to reach the heart.

Heart bypass surgery begins with an incision in the chest, and the breastbone is cut exposing the heart. Next, a portion of the saphenous vein, which is very large, is harvested from the inside of the leg. Pieces of this large vein are used to bypass the blocked coronary arteries, which are arteries that supply blood to the heart. The venous graft is sewn to the aorta, the main artery of the body, and to the affected coronary artery, to bypass the blocked site.

The internal mammary artery from the chest may also be used to bypass a clogged artery.

Several arteries may be bypassed depending on the condition of the heart. After the graft is created, the breastbone and chest are closed.

The embryo’s heart is the first organ that forms in its tiny body, and like most complex instruments, it begins with some simple structures.

Let’s go back to 18 days after conception...Looking in the mother’s uterus, you can see the embryo surrounded by its yolk sac and amnion. Let’s take a look inside.

Here’s a diagram of the embryo seen from a side view. Right now, it’s about the size of a raisin. There’s the head region and that red-colored area slightly above it contains two tubes that will form the embryo’s heart. Here’s what the tubes look like from a front view.

On day 21, we see that the primitive heart tubes have moved below the embryo’s developing head region. And by day 22, the tubes have fused together, and have moved to the area that will eventually be our embryo’s thoracic, or chest cavity. It’s also about this time that the heart begins to beat for the first time...

Here’s what it looks like from the front.

Now let’s go back to day 18 and watch this happen from a different viewpoint. Here are two tubes in our embryo’s chest region seen from a front view. Watch this... Over the next two days, these tubes fuse together.

Here’s another amazing part: the tube now starts bending and twisting and over the next 8 days it forms a simple version of the heart.

By the time the embryo becomes a fetus at two months, the heart bears a close resemblance to what it will look like after the baby’s born. But the resemblance is only superficial. On the inside of the heart, things are much different in both form and function.

Here’s a newborn heart on the left. Let’s take a closer look. There’s the right atrium right ventricle, left atrium and left ventricle. The two major blood vessels are the aorta and the pulmonary artery.

The pathway of blood in the newborn heart works like this: oxygen-poor blood from the body enters the right atrium, then goes to the right ventricle. From the right ventricle, the blood is pumped to the lungs where it becomes oxygen rich. Then the blood flows back to the heart filling the left atrium and from there on to the left ventricle. The left ventricle pumps the oxygen rich blood through the aorta, which carries it to the rest of the newborn’s body.

You can see the fetal heart has the same basic components as the newborn heart, but there are a couple important differences. Because the placenta is providing all of the oxygen the fetus requires, its lungs are not needed to perform this task, and therefore much of the fetus’ blood is detoured away from the lungs through two openings or connections. They are the foramen ovale, which connects the right and left atria, and the ductus arteriosus which connects the aorta and the pulmonary artery.

As blood enters the heart into the right atrium some of the blood flows into the right ventricle as in the newborn, but also notice that some blood flows directly into the left atrium through the foramen ovale. This blood will pass directly into the left ventricle and be pumped out to the body without ever having gone to the lungs. In addition, some of the blood that did enter the right ventricle, and would normally go to the lungs, never reaches the lungs.

Here lets watch. As blood is being pumped out of the right ventricle towards the lungs through the pulmonary artery, some of that blood escapes into the aorta through the ductus arteriosus, bypassing the lungs as it does. These two important connections will remain open up until the time of birth.

Within thirty minutes after the baby’s first breath, the ductus arteriosus will completely close, and the flap of the foramen ovale will shut off like a valve. This happens because of an increase in pressure on the left side of the heart, and a decrease on the right side. These changes in the heart anatomy cause the blood to flow to the lungs, which will take over their lifelong job of supplying oxygen to the body.

It’s incredible to think that this complex organ started off as a couple of tubes only 2 1/2 weeks ago.

The heart has four chambers and four main blood vessels that either bring blood to the heart, or carry blood away.

The four chambers are the right atrium and right ventricle and the left atrium and left ventricle. The blood vessels include the superior and inferior vena cava. These bring blood from the body to the right atrium. Next is the pulmonary artery that carries blood from the right ventricle to the lungs. The aorta is the body's largest artery. It carries oxygen-rich blood from the left ventricle to the rest of the body.

Beneath the tough fibrous coating of the heart, you can see it beating.

Inside the chambers are a series of one-way valves. These keep the blood flowing in one direction.

Dye injected into the superior vena cava, will pass through all the heart's chambers during one cardiac cycle.

Blood first enters the heart's right atrium. A muscle contraction forces the blood through the tricuspid valve into the right ventricle.

When the right ventricle contracts, blood is forced through the pulmonary semilunar valve into the pulmonary artery. Then it travels to the lungs.

In the lungs, the blood receives oxygen then leaves through the pulmonary veins. It returns to the heart and enters the left atrium.

From there, blood is forced through the mitral valve into the left ventricle. This is the muscular pump that sends blood out to the rest of the body.

When the left ventricle contracts, it forces blood through the aortic semilunar valve and into the aorta.

The aorta and its branches carries the blood to all the body's tissues.

Eating spicy foods, such as pizza, may cause a person to feel heartburn.

Although the name may imply the heart, heartburn has nothing to do with the heart itself. Heartburn is pain felt in the chest by a burning sensation in the esophagus.

Here, you can see the pizza passing from the mouth to the esophagus and on to the stomach.

At the junction between the stomach and esophagus is the lower esophageal sphincter. This muscular sphincter acts as a valve that normally keeps food and stomach acid in the stomach, and prevents the stomach’s contents from regurgitating back into the esophagus.

However, certain foods may affect the lower esophageal sphincter, making it less effective. That’s how heartburn begins.

The stomach produces hydrochloric acid to digest food. The stomach has a mucous lining that protects it from hydrochloric acid, but the esophagus does not.

So, when food and stomach acid regurgitate back into the esophagus, a burning feeling is felt near the heart. This feeling is known as heartburn.

Antacids may be used to relieve heartburn by making stomach juices less acidic, thereby reducing the burning feeling felt in the esophagus. If heartburn becomes frequent or prolonged, medical intervention may be necessary to correct the problem.

The disks between the vertebrae are liable to displacement when put under strain. Heavy lifting may produce forces which cause a lumbar intervertebral disk to move out of place ("slipped disk").

Homeostasis is a state of balance inside the body, where the body systems work together to keep it functioning normally. The endocrine system keeps this internal balancing act going by releasing chemicals called hormones. The release of the hormones is controlled by negative feedback mechanisms.

A negative feedback mechanism works something like a thermostat in your home. A thermostat helps maintain a constant temperature. Think of that as the normal range. When the temperature rises beyond the normal range, the thermostat turns on the air conditioner. Eventually the air conditioner restores the temperature to the normal range. That’s negative feedback. It starts with a normal range, goes beyond the normal range, and then back to normal again.

Now, let’s look at how this mechanism works in the body.

One example is how the endocrine system controls the amount of sugar in the bloodstream. Insulin is a hormone secreted by the pancreas that maintains a normal amount of sugar in the bloodstream. Shortly after eating a candy bar, tiny sugar molecules enter the bloodstream raising the blood sugar levels. In response, the pancreas secretes the insulin into the bloodstream. Now, the sugar molecules move out of the bloodstream and into the cells of the skeletal muscles, fat and liver. In turn, the blood sugar levels return to normal.

That’s how a negative feedback mechanism works to maintain the body’s internal balance.

You might not be aware of this, but during its early development a fetus looks remarkably like something from the dawn of time.

Here, let’s take a closer look. There’s a human fetus’s head during the first month of development, when it was still an embryo. Its face starts as a series of paired tissue mounds called branchial arches.

Let’s take a look from the front. The embryo’s face actually forms from the first branchial arch, along with the area just above it. The forehead and nose form from this area. These areas will form the cheekbones, and these lower areas will form the lower jaw. And this area will form the mouth. At 28 days of development, you can see the lower jaw, which has fused together from the branchial arches. The thickenings you see here will eventually form the nostrils.

By day 31, you can see the nostrils have started to form. And, quite remarkably, the eyes have now appeared on each side of the head.

Two days later, the nostrils have moved toward the center of the face. You can also see that as the ears begin to form, they are positioned in a pretty odd location. But don’t worry, they will move.

At 35 days, the nostrils are even closer together, and we can see more of the eyes. At 40 days, the baby has developed eyelids, and the nose looks much more developed.

Here he is at 48 days and he’s looking pretty darn good. The nasal swellings have joined in the center of the face, and the eyes have moved to the front of the head.

Three weeks later, the fetus looks more human than ever. After that, its face continues to develop more typical proportions right up until the time of its birth.

Let’s look at the entire process again...

As you can see, the development of the face is a fascinating process that has some very dramatic changes taking place in a relatively short amount of time.

If left untreated, hypertension can lead to a thickening of arterial walls, causing the lumen, or blood passageway, to narrow in diameter. As a result, the heart must work harder to pump blood through the narrowed arterial openings. In addition, people with hypertension may be more susceptible to stroke.

Special white blood cells called lymphocytes play a key role in the immune system's response to foreign invaders. There are two main groups, both of which form in bone marrow.

One group, called T-lymphocytes or T-cells, migrates to a gland called the thymus.

Influenced by hormones, they mature there into several types of cells, including helper, killer, and suppressor cells. These different types work together to attack foreign invaders. They provide what's called cell-mediated immunity, which can become deficient in persons with HIV, the virus that causes AIDS. HIV attacks and destroys helper T cells.

The other group of lymphocytes are called B-lymphocytes or B cells. They mature in the bone marrow and gain the ability to recognize specific foreign invaders.

Mature B cells migrate through the body fluids to the lymph nodes, spleen, and blood. In Latin, body fluids were known as humors. So B-cells provide what's known as humoral immunity. B-cells and T-cells both circulate freely in blood and lymph, searching for foreign invaders.

Intracytoplasmic sperm injection, or ICSI, is a form of in vitro fertilization. That means the egg is fertilized outside the body. First, egg cells are harvested. Then they're placed in a special media in a laboratory dish.

Within a few hours, a sperm is injected through a fine needle into the center of an egg. If successful, the cell will divide and form the first stages of an embryo.

Typically, several eggs are harvested and fertilized at the same time. Then they're placed in the uterus. This increases the chance one will implant and become a successful pregnancy.

Before we talk about how kidney stones are formed, take a moment to become familiar with the urinary tract.

The urinary tract includes the kidneys, ureters, bladder, and urethra.

Now let’s enlarge a kidney to get a closer view. Here’s a cross-section of the kidney. Urine flows from the outer cortex to the inner medulla. The renal pelvis is the funnel through which urine exits the kidney and enters the ureter.

As urine passes through the kidneys, it can become very concentrated. When the urine becomes too concentrated, calcium, uric acid salts, and other chemicals dissolved in the urine can crystallize, forming a kidney stone, or renal calculus.

Usually the calculus is the size of a small pebble. But ureters are very sensitive to being stretched, and when stones form and distend it, the stretching can be very painful. Often, people may not know they have kidney stones until they feel the painful symptoms resulting from a stone being stuck anywhere along the urinary tract. Fortunately, small stones typically pass out of the kidneys and through the ureters on their own, without causing any problems.

However, stones can become more problematic when they block the flow of urine. Doctors call this one a staghorn kidney stone, and it is obstructing the entire kidney. Fortunately, these stones are the exception rather than the rule.

Emily: How does the baby come out?

Mommy: When my baby is ready to be born, I start to feel labor contractions. That means that my uterus starts squeezing and pushing so the baby can come out. Here you can see the baby coming out of my uterus. It’s a tight fit, but it doesn’t hurt the baby one bit.

Emily: Mommy, how big is the baby in your uterus?

Mommy: Well Emily, the baby is only been growing inside my uterus for one month, so he or she is very small. Here, let me show you on the computer. Why, right now the baby could fit in the palm of your hand.

Emily: Is she as small as my tinker-tot doll?

Mommy: She’s even smaller. After a month in my uterus, the baby is about the size of your thumbnail.

Emily: Wow…

Mommy: And this is what the baby looks like up-close.

Emily: She’s cute. But Mommy, she doesn’t have any ears or toes.

Mommy: Don’t worry dear, the baby is still very young, so he or she still needs to grow some more.

Emily: Will she get her ears and toes then?

Mommy: Yes, honey. Here, this is what the baby looks like inside my uterus. See?

Emily: There she is! She’s tiny.

Mommy: Yes. But watch what happens over the next 9 months.

Emily: Wow, that will be fun! Then I can play with my baby sister!

Emily: Mommy, will the baby be a girl or a boy?

Mommy: Well, remember when we talked about Daddy’s sperm meeting my egg cell and making a baby?

Emily: Yes. That happened in your uterus?

Mommy: Well… that’s close, Emily. But they actually met in one of my fallopian tubes, which connects to my uterus. And here’s something else. Inside Daddy’s sperm are a set of instructions that tell the baby to be a boy or a girl.

Emily: Instructions… like the kind we used to build my dollhouse?

Mommy: Yes, except these instructions are for building a baby.

Emily: Gee… I bet that’s harder than building a dollhouse.

Mommy: It sure is. And here’s something very important. The instructions in Daddy’s sperm have a secret letter on them.

Emily: A secret letter?

Mommy: Yes. And if the letter is an “X”, it means the baby will be a girl like you.

Emily: “X”… “X” marks the spot!

Mommy: That’s right! And if the letter is a “Y”… what do you think that means?

Emily: “Y”… why it will be a boy.

Mommy: Right! “X” is for girls… “Y” is for boys.

Emily: I sure hope it’s an “X”.

Emily: How does the baby eat and breathe in your uterus?

Mommy: When the baby’s in my uterus, it can’t eat soup or breathe on its own, so it needs some help. The baby has a little tube that goes to its middle. It’s called the umbilical cord. And the umbilical cord goes to the placenta, which connects to my uterus. Here’s how it works. First, the food that I eat and air that I breathe get into my bloodstream. Emily, did you know that in my blood there are tiny pieces of food and air so small, that you can’t even see them with a microscope?

Emily: No, I didn’t.

Mommy: It’s true! And they’re in yours, too. These tiny pieces of food and air travel along my bloodstream to the placenta. And from there, they go to the umbilical cord and into the baby’s body. That’s how the baby eats and breathes inside my uterus.

Emily: He’s like a little deep-sea diver.

Mommy: That’s right, Emily. And did you know after the baby is born, the umbilical cord goes away? And guess what’s left.

Emily: What?

Mommy: You’re belly button!

Emily: (Giggle)

Emily: Mommy, where do babies come from?

Mommy: You need two things to make a baby: a sperm cell and an egg cell. Daddy makes the sperm cell inside his body and I make the egg cell inside my body. They don’t look so small here, but you would need a microscope to see them in real life. A microscope is sort of like a magnifying glass, only much stronger. When the sperm cell and the egg cell meet each other, they make a tiny baby that’s smaller than a grain of salt.

Emily: Wow… that’s small.

Mommy: It sure is, and the baby will grow inside a special place in my tummy called my uterus. And then, the baby will come out as your little baby brother or sister.

Emily: I can hardly wait! I hope it’s a sister.

A vibrating suction cannula, or flexible tube, is used in a liposuction procedure. In this example, the cannula is inserted through a small hole into the abdominal fat tissue to remove excess fat deposits.

The lymphatic system has two main functions. Its network of vessels, valves, ducts, nodes, and organs helps balance the body's fluid by draining excess fluid, known as lymph, from the body's tissue and returning it to the blood after filtering it. Some types of blood cells are also made in the lymph nodes.

The lymphatic system also plays an important role in the body's immune system. Infection, even a trivial infection is, the most common cause of swollen lymph nodes.

Let's look at a cut section of a lymph node to see what happens.

Afferent means towards. Afferent lymph vessels bring unfiltered fluids from the body into the lymph node where they are filtered.

Efferent vessels, meaning away from, carry the clean fluid away and back to the bloodstream where it helps form plasma.

When the body is invaded by foreign organisms, the swelling sometimes felt in the neck, armpits, groin, or tonsils comes from the microorganisms trapped inside the lymph nodes.

Eventually, these organisms are destroyed and eliminated by cells that line the node walls. Then the swelling and pain subside.

The body is mostly composed of fluids. All its cells contain and are surrounded by fluids. In addition, four to five liters of blood circulate through the cardiovascular system at any given time. Some of that blood escapes from the system as it passes through tiny blood vessels called capillaries in the body tissues. Fortunately, there is a "secondary circulatory system" that reabsorbs escaped fluid and returns it to the veins.

That system is the lymphatic system. It runs parallel to the veins and empties into them. Lymph forms at the microscopic level. Small arteries, or arterioles, lead to capillaries, which in turn lead to small veins, or venules. Lymph capillaries lie close to the blood capillaries, but they are not actually connected. The arterioles deliver blood to the capillaries from the heart, and the venules take blood away from the capillaries. As blood flows through the capillaries it is under pressure. This is called hydrostatic pressure. This pressure forces some of the fluid in the blood out of the capillary into surrounding tissue. Oxygen from the red blood cells, and nutrients in the fluid then diffuse into the tissue.

Carbon dioxide and cellular waste products in the tissue diffuse back into the bloodstream. The capillaries reabsorb most of the fluid. The lymph capillaries absorb what fluid is left.

Edema, or swelling, occurs when fluid in or between the cells leaks into the body tissues. It is caused by events that increase the flow of fluid out of the bloodstream or prevent its return. Persistent edema may be a sign of serious health problems and should be checked by a health care professional.

The lymphatic system can play a very worrisome role in the spread of breast cancer.

Lymph nodes filter the lymph as it passes through the system. They are located at specific points throughout the body such as in the armpits and high in the throat.

Lymphatic circulation in breast tissue helps regulate the local fluid balance as well as filter out harmful substances. But the breast's lymphatic system can also spread diseases such as cancer through the body.

Lymphatic vessels provide a highway along which invasive cancerous cells move to other parts of the body.

The process is called metastasis. It can lead to the formation of a secondary cancer mass in another part of the body.

This mammogram shows a tumor and the lymph vessel network it has invaded.

No woman is too young to know that regular breast self-examinations can help to catch tumors earlier in their growth, hopefully before they spread or metastasize.

The macula is the part of the retina that distinguishes fine details at the center of the field of vision.

As we enlarge to a cross-section, macular degeneration is better seen as the partial breakdown of the insulating layer between the retina and the choroid layer of blood vessels behind the retina.

Click and drag the slider bar to see changes that occur during a normal 28 day menstrual cycle.

Muscles perform four important body functions. They maintain body posture, they stabilize the joints, they provide mobility, enabling us to move where we want, and they generate heat that the body requires.

The body contains three types of muscle to perform these functions: skeletal muscle, cardiac muscle, and smooth muscle. Smooth muscle is found in the walls of body organs, like the stomach and small intestines. It is also called involuntary muscle because it functions without conscious directions. If smooth muscle required conscious directions, we would have to focus our concentration on functions like digesting our food and passing it through the entire length of our small and large intestines.

Another involuntary muscle group, cardiac muscle, is found only in the heart, where it makes up the heart wall. It has its own specialized electrical conduction system. What this microscopic view doesn’t show you is that all of the separate muscle fibers act in concert, so that the heart contracts and relaxes as if it were one enormous muscle. If cardiac muscle needed conscious instructions to contract and relax, we would have to think about every heartbeat.

The third type of muscle is skeletal muscle, and it is the only type under voluntary control. Skeletal muscle attaches to and covers our bony skeleton. In this microscopic view, you can see that it has a series of dark and light stripes, or striations. Because of this, it is often called striated muscle.

Regular workouts that include resistance training, such as weight lifting, can result in an increase in skeletal muscle size. It’s important to realize that skeletal muscles are under voluntary control. Otherwise, we would not be able to control the body’s movements.

The nervous system is made up of two parts. Each part contains billions of neurons. The first part is the central nervous system. It contains the brain and spinal cord, which is a fibrous, ropelike structure that runs through the spinal column down the middle of the back.

The other part is the peripheral nervous system. It consists of thousands of nerves that connect the spinal cord to muscles and sensory receptors. The peripheral nervous system is responsible for reflexes, which help the body avoid serious injury. It's also responsible for the fight or flight response that helps protect you when you feel stress or danger.

Let's examine an individual neuron up close.

Here is a peripheral nerve. Each one of the nerve bundles, or fascicles, contains hundreds of individual nerve.

Here's an individual neuron, with its dendrites, axon, and cell body. The dendrites are tree-like structures. Their job is to receive signals from other neurons and from special sensory cells that tell us about our surroundings.

The cell body is the headquarters of the neuron. It contains the cell's DNA. The axon transmits signals away from the cell body to other neurons. Many neurons are insulated like pieces of electrical wire. The insulation protects them and allows their signals to move faster along the axon. Without it, signals from the brain might never reach muscle groups in the limbs.

Motor neurons are responsible for voluntary control of the muscles all over the body. The operation of the nervous system depends on how well neurons communicate. For an electrical signal to travel between two neurons, it must first be converted to a chemical signal. Then it crosses a space about a millionth of an inch wide. The space is called a synapse. The chemical signal is called a neurotransmitter.

Neurotransmitters allow the billions of neurons in the nervous system to communicate with one another. That's what makes the nervous system the body's master communicator.

The most critical stage of development for the embryo’s nervous system is the third and fourth weeks of pregnancy. Starting with the uterus, let’s enlarge the area where the embryo has implanted. Here’s the embryo implanted in the uterus on day 14. You can see that it lies within the wall of the uterus, and is covered by a single layer of cells.

Take a moment to get oriented. There’s the yolk sac, which makes blood cells for the embryo, and the amnion, which surrounds and protects the embryo, and the blood vessels that will help form the placenta. Also notice that the embryo is connected to the uterus by a small connecting stalk, which will eventually become the umbilical cord.

If we rotate it to the left we’ll get a better view of the back of the embryo, where we can see the brain and spinal cord develop.

On day 14, the embryo looks like a little disc. The first part of the nervous system that forms is an indentation called the neural groove. Over the next seven days, the groove deepens as the cells around it form ridges called neural folds.

By day 27 we see that the neural folds wrap around the neural groove and form the neural tube. The neural fold will become the spinal cord. Those bundles of cells that look like building blocks are called somites. They form the vertebral column, or backbone. They also help form the ribs, and the muscles of the neck, arms, and legs.

Now let’s watch it again without interruption…

If we rotate the embryo again to the left, we can see the areas that will eventually become the brain and spinal cord. The embryo’s nervous system is particularly vulnerable at this stage of development, so an expectant mother should be careful about avoiding any substances that could potentially harm it.

Red blood cells are the oxygen carriers. As they travel away from the heart, they traverse smaller and smaller arteries, finally arriving at the collections of microscopic blood vessels, known as capillaries. Here, they exchange nutrients and oxygen for cellular waste products. The exchange of oxygen and nutrients between the red blood cells and the surrounding tissues occurs through a process called diffusion.

In diffusion, when capillaries contain a high concentration of oxygen and nutrients, while the surrounding tissues contain a lower concentration, oxygen and nutrients leave the capillaries and enters the tissues.

Conversely, when body tissues contain high concentrations of carbon dioxide and metabolic waste, while the capillaries contain a lower concentration, the waste products diffuse from the tissues into the capillaries, and from there are carried by the venous system back toward the heart.

The waste products are eventually eliminated from the bloodstream through the urinary and respiratory systems."

Osteoarthritis is the most common form of arthritis and is associated with the aging process.

Even from the outside, you can see that the knee of an older person looks considerably different than that of a younger person.

Let's take a look at the joint itself to see the differences.

Osteoarthritis is a chronic disease, a disease that persists for a long time. It causes the deterioration of the cartilage within a joint. For most people, the cause of osteoarthritis is unknown, but metabolic, genetic, chemical, and mechanical factors play a role in its development.

Symptoms of osteoarthritis include loss of flexibility, limited movement, and pain and swelling within the joint. The condition results from injury to the cartilage, which normally absorbs stress and covers the bones, so they can move smoothly. The cartilage of the affected joint is roughened and becomes worn down. As the disease progresses, the cartilage becomes completely worn down and the bone rubs on bone. Bony spurs usually develop around the margins of the joint.

Part of the pain results from these bone spurs, which can restrict the joint's movement as well.

This elderly woman had to be taken to the hospital last night. While getting out of the tub, she had a fall and broke her hip. Because her bones are so fragile, the woman probably broke her hip first, which then caused her to fall.

Like millions of people, the woman suffers from osteoporosis, a condition that leads to loss of bone mass.

From the outside, osteoporotic bone is shaped like normal bone. But the inside appearance of the bone is quite different. As people age, the inside of the bones becomes more porous, due to the loss of calcium and phosphate. The loss of these minerals makes the bones more prone to fracture, even during routine activities, like walking, standing, or bathing. Many times, a person will sustain a fracture before becoming aware of the presence of the disease.

Prevention is the best measure for treating osteoporosis by eating a recommended balanced diet including foods with sufficient amounts of calcium, phosphorous, and vitamin D. In addition, maintaining a regular exercise program as approved by a qualified health care professional will help to keep the bones strong.

Various medications can be used as part of the treatment for osteoporosis and should be discussed with a qualified health care professional.

Ovulation starts when the pituitary gland releases the hormone FSH to start the development of an egg.

LH, another pituitary hormone, helps the egg mature then triggers its release.

Let's see what happens once it leaves the ovary.

Fimbriae form a finger-like fringe on the end of the fallopian tube. Their waving motion draws the egg across a small space in the body cavity and into the tube.

There, it will either be fertilized by a sperm or dissolve if it's not.

In Parkinson disease, the production of a brain chemical called dopamine becomes irregular and inadequate, and nerve cells cannot properly transmit messages. This results in the loss of muscle function.

By providing an even, adequate supply of medication that the body converts into dopamine, neurons are able to transmit messages, and tremors decrease.

PTCA, or percutaneous transluminal coronary angioplasty, is a minimally invasive procedure that opens blocked coronary arteries to improve blood flow to the heart muscle.

First, a local anesthesia numbs the groin area. Then, the doctor puts a needle into the femoral artery, the artery that runs down the leg. The doctor inserts a guide wire through the needle, removes the needle, and replaces it with an introducer, an instrument with two ports for inserting flexible devices. Then the original guide wire is replaced by a thinner wire. The doctor passes a long narrow tube called a diagnostic catheter over the new wire, through the introducer, and into the artery. Once it's in, the doctor guides it to the aorta and removes the guide wire.

With the catheter at the opening of a coronary artery, the doctor injects dye and takes an X-ray.

If it shows a treatable blockage, the doctor backs the catheter out and replaces it with a guiding catheter, before removing the wire.

An even thinner wire is inserted and guided across the blockage. A balloon catheter is then guided to the blockage site. The balloon is inflated for a few seconds to compress the blockage against the artery wall. Then it's deflated. The doctor may inflate the balloon a few more times, each time filling it a little more to widen the passage.

This may then be repeated at each blocked or narrowed site.

The doctor may also place a stent, a latticed metal scaffold, within the coronary artery to keep it open.

Once the compression is done, dye is injected and an X-ray is taken to check for changes in the arteries.

Then the catheter is removed and the procedure is complete.

Peristalsis is a series of wave-like muscle contractions that move food through the digestive tract. It starts in the esophagus where strong wave-like motions of the smooth muscle move balls of swallowed food to the stomach. There, the food is churned into a liquid mixture called chyme that moves into the small intestine where peristalsis continues.

Stretching out a piece of intestine will make it easier to see the wave-like motion. The motion mixes and shifts the chyme back and forth. This lets the bloodstream absorb nutrients through the walls of the small intestine.

In the large intestine peristalsis helps water from undigested food be absorbed into the blood stream. Then, the remaining waste products are excreted through the rectum and anus.

These amoeba-like cells are a type of phagocyte called macrophages. They're scavenger cells that can form tentacles called pseudopods to surround and ingest foreign cells.

Once swallowed, the cells are walled off and destroyed by a bag of digestive enzymes.

The pituitary gland lies deep inside the head. It's often called the "master gland" because it controls many of the things other glands do.

Just above the pituitary is the hypothalamus. It sends hormonal or electrical signals to the pituitary. These determine which hormones the pituitary will release.

For example, the hypothalamus might send a hormone called GHRH, or growth hormone releasing hormone. That would trigger the pituitary's release of growth hormone, which affects the size of both muscle and bone.

How important is this? Not getting enough during childhood can cause pituitary dwarfism. Getting too much can cause the opposite condition called gigantism. In a body that has already matured, too much growth hormone can cause acromegaly. With this condition, facial features become rough and course; the voice becomes deeper; and hand, foot, and skull size expand.

A different hormonal command from the hypothalamus might trigger the release of thyroid stimulating hormone or TSH. TSH causes the thyroid to release two hormones called T3 and T4 that stimulate metabolism in other cells throughout the body.

The pituitary can also release a hormone called antidiuretic hormone, or ADH. It's produced in the hypothalamus and stored in the pituitary. ADH affects the production of urine. When it's released, the kidneys absorb more of the fluid that passes through them. That means less urine is produced.

Alcohol inhibits the release of ADH, so drinking alcoholic beverages results in more urine production.

The pituitary gland produces other hormones that control other bodily functions and processes.

For instance, follicle stimulating hormone, or FSH, and luteinizing hormone, or LH, are hormones that affect the ovaries and egg production in women. In men, they affect the testes and sperm production.

Prolactin is a hormone that affects breast tissue in nursing mothers.

ACTH or adrenocorticotrophic hormone causes the adrenal glands to produce important substances similar to steroids.

Growth, puberty, baldness, even sensations like hunger and thirst, are just a few of the processes that are influenced by the endocrine system.

The placenta provides the baby with nutrients and oxygen from the mother and carries away fetal waste. Following delivery, the uterus naturally contracts to push the placenta out of the uterus. In addition, the delivering practitioner will assist by gently pressing the abdomen to work the placenta free of the uterus.

Delivery of the placenta is typically painless and takes approximately 15 minutes.

Once the placenta is delivered, it is examined to see if the placental tissue is healthy and in one piece. At times, the placenta can break off and cause bleeding in the uterus, but as we see here, the placenta is delivered in one piece perfectly.

The placenta is commonly referred to as the afterbirth. Its successful delivery signals the end of the final stage of childbirth.

Many people have mistaken ideas about how a growing embryo eats and breathes in the uterus.

From the earliest stages of its development, the growing embryo requires nutrition and oxygen, and a disposal system for the waste products of its own metabolism. All of this is accomplished by the placenta, which allows the growing embryo to eat and breathe while in the mother's uterus.

To get some perspective on how the placenta began, let's go back to Day 8. This hollow ball of cells moving through the uterus is the blastocyst, searching for an implantation site. Here you see its outer layer beginning to extend out and implant in the uterine lining, searching for the uterine blood vessels that would nourish it throughout the pregnancy.

As it went deeper, a single layer of cells from the mother's uterine lining surrounded it, so that it would be protected from harm. On Day 9, as it grew larger and more complex, the blastocyst became an embryo. Here it's about the size of a pinhead.

Also on Day 9, the outer layer of the embryo developed spaces called lacunae. The lacunae filled up with blood from the mother's uterine lining.

On Day 13, small projections from the embryo's chorionic layer reached out into the uterine lining. The chorionic layer is one of the membranes that surround the embryo and help it implant.

On Days 15 through 21, blood vessels began to form beneath this chorionic layer.

Around Day 21, the embryo's blood stream and the mother's blood stream were in such close contact that nutrients and oxygen could cross from mother to embryo. This was how the embryo first got its food and air from the mother, and technically this is when the placenta began to function.

Let's magnify this area so you can see what we're talking about. Here you see a vein and an artery from the embryo in close contact with the blood in the mother's uterine lining. Inside the blood vessels, you can also see red blood cells, which carry oxygen.

The two blood streams are separated by a thin collection of tissues in the placenta called the blood barrier. This barrier permits small particles like nutrients and oxygen to pass from the mother to the embryo, and allows waste products to pass from the embryo back to the mother. The blood barrier also prevents many large or potentially harmful particles from entering the embryo's blood stream. Notice that the red blood cells do not cross from the mother's blood stream to the embryo's.

You may be wondering how a mother's blood cells could be harmful to her growing baby, and why it's important to keep the two blood streams separate. If the mother's blood type is RH negative, and her embryo's blood type is RH positive, then the mother's antibodies would treat the embryo as an invading foreign organism, and try to destroy it.

Now you can see why the placenta and its blood barrier are important for supplying the growing embryo with nutrition and oxygen, removing its waste products, and preventing harmful substances from getting into its blood stream.

High blood pressure from preeclampsia can put the fetus at risk. As the pressure narrows placental arteries, some areas of the placenta may not get enough blood. Over time, areas of the placenta could "die", putting the baby in distress.

The first trimester consists of the first 14 weeks of the 40-week pregnancy. During this time, a woman may experience various emotions due to the hormonal changes in her body, and she may experience “morning sickness” at any time during the day or night.

During the first trimester, nearly all of the fetus’ internal organs form. The fetus is approximately the size of a fist and can even begin to move a little.

During the second trimester, weeks 15 through 27, a woman may experience symptoms of heartburn and indigestion. The baby’s growth in the abdominal area becomes noticeable, and stretch marks may develop as the skin of the abdomen expands. The developing fetus grows very rapidly during the second trimester, and the fetus’ arms and legs become well developed and strong. The expectant mother may begin to feel the baby moving inside. Although the fetus is almost fully formed, the lungs need to develop further while the fetus puts on more fat and weight.

During the third trimester, weeks 28 through 40, the baby continues to grow and the lungs develop further. It can be difficult for a woman at this stage to find a comfortable position either awake or asleep.

The baby’s growing body within the uterus pushes the abdominal organs up under the diaphragm and compresses the bladder and colon. Near the end of the 40-week cycle, the baby drops down into the pelvic cavity, which is nature’s way of saying, “It’s time to give birth”.

Blood has been called the river of life, transporting various substances that must be carried to one part of the body or another. Red blood cells are an important element of blood. Their job is to transport oxygen to the body's tissues in exchange for carbon dioxide, which they carry to the lungs to be expelled. Red blood cells are formed in the red bone marrow of bones. Stem cells in the red bone marrow are called hemocytoblasts. They give rise to all of the formed elements in blood.

If a stem cell commits to becoming a cell called a proerythroblast, it will develop into a new red blood cell.

The formation of a red blood cell takes about 2 days. The body makes about two million red blood cells every second!

Blood is made up of both cellular and liquid components. If a sample of blood is spun in a centrifuge, the formed elements and fluid matrix of blood can be separated from each other.

Blood consists of 45% red blood cells, less than 1% white blood cells and platelets, and 55% plasma.

Most of the time, the skeletal muscles are under voluntary, or conscious, control. However, their movement can also be induced by involuntary reflexes. Reflexes are involuntary reactions to a stimulus. In this example, the sensation of hot oatmeal on the skin causes this man’s hand to quickly withdraw to prevent further injury.

Let’s look at an instant replay to see how this reflex response works.

As soon as the hot oatmeal contacts the hand, the pain receptors send a signal to the spinal cord. In turn, the spinal cord sends a signal back to the arm muscles that make him pull his hand away. Because the arm flexed as it withdrew, this reflex is really known as a flexor, or withdrawal, reflex. There are many other reflexes that protect the body as well.

If this man didn't have the reflexes to let him withdraw quickly from a painful stimulus, he would be at risk for serious injury.

As light enters the eye, it strikes receptor cells on the retina called rods and cones. The light triggers a chemical reaction in the cells, which then send electric impulses through the optic nerve to the brain.

Retinal detachments are associated with a tear or hole in the retina through which the internal fluids of the eye may leak, causing separation of the retina from underlying tissues.

One of the first signs of labor starting is the appearance of a mucus plug, or what is sometimes called a "bloody show".

The bloody show is the discharge of a small amount of pinkish mucus that formed the barrier between the uterus and vagina during pregnancy.

Shortly after the bloody show, another important event occurs: the amniotic sac ruptures and amniotic fluid begins to trickle out of the uterus and vagina. For some women, it can actually gush out in a stream.

The rupturing of the amniotic sac, which surrounds and protects the baby, is commonly referred to as the "water breaking".

The combination of contractions, the bloody show, and water breaking indicates the first phase of labor starting.

Vision is the dominant sense for most people with sight.

The organ of sight is the eye. Think of it as a slightly irregular, hollow sphere that takes in light and translates it into images.If we enlarge the eye and look inside it, we can discover how that's done.

Inside the eye are various structures working together to create an image the brain can understand. Among these are the cornea, a clear dome-like structure covering the iris or colored part of the eye, the lens directly below it, and the retina, which lines the back of the eye. The retina consists of thin layers of light-sensitive tissue.

This candle can help us understand how the eye captures images and then sends them to the brain. First, the candlelight passes through the cornea. As it does, it's bent, or refracted, onto the lens. As the light passes through the lens, it's bent a second time. Finally, it arrives at the retina where an image is formed.

This double bending, though, has reversed the image and turned it upside down. If that was the end of the story, the world would always appear upside down. Fortunately, the image is turned right side up in the brain.

Before that can happen, the image needs to travel as impulses along the optic nerve and enter the brain's occipital lobe. When the image forms there, it regains its proper perspective.

Now let's consider two common conditions that cause blurry vision. The eye's shape is important for keeping things in focus. With normal vision, light focuses precisely on the retina at a location called the focal point.

But what happens if the eye is longer than normal? The longer the eye, the more distance there is between the lens and retina. But the cornea and lens still bend light the same way. That means the focal point will be somewhere in front of the retina rather than on it.

This makes it difficult to see things that are far away. A person with a long eye is said to be nearsighted. Glasses with concave lenses can correct nearsightedness.

The lens widens the plain of light coming through the cornea. That pushes the focal point back onto the retina.

Farsightedness is just the opposite. The eye's length is too short. When that happens, the focal point is behind the retina. So it's difficult to see things that are up close.

Glasses with convex lenses narrow the plain of light. Narrowing the light passing through the cornea moves the focal point back onto the retina and can correct farsightedness.

A baby’s sex is determined at the time of conception. When the baby is conceived, a chromosome from the sperm cell, either X or Y, fuses with the X chromosome in the egg cell, determining whether the baby will be female or male. Two X’s means the baby will be a girl, and XY means it will be a boy.

But even though gender is determined at conception, the fetus doesn’t develop its external sexual organs until the fourth month of pregnancy.

Let’s go to seven weeks after conception. You can see from the front that the fetus appears to be sexually indifferent, looking neither like a male or a female.

Over the next five weeks, the fetus begins producing hormones that cause its sex organs to grow into either male or female organs. This process is called sexual differentiation.

We don’t know what sex this fetus is yet, so we’ll have to be hypothetical here....Now, if the fetus is a male, it will produce hormones called androgens, which will cause his sexual organs to form like this.

On the other hand, a female fetus would not produce androgens; she would produce estrogens… so her sex organs would form like this.

Now let’s take a look at something you may have missed. At seven weeks, the sex organs of a male and female look identical. Let’s add some color to see what happens during sexual differentiation. Keep your eye on the genital tubercle.

See that? The genital tubercle formed the penis in the male, and the clitoris in the female.

The penis and clitoris are called sexual analogs because they originate from the same structure.

Force on a joint can cause a dislocation. The force pushes the bone out of the socket, which may damage surrounding ligaments, tendons, and nerves.

A baby’s skeleton begins as fragile membranes and cartilage, but after three months, the membranes and cartilage start turning into bone, providing protection for the internal organs, and a solid framework for the muscles.

Late in the second month of fetal development, a fetus’ skeleton is made up of thin membranes, which are about the thickness of paper tissue, and soft, flexible cartilage, like the kind you find in your ear. Over time, both types of tissue will turn into bone in a process called ossification.

Ossification occurs in two ways...the first is when membranes turn into bone.

If we look at a fetus during the third month, we can see that the membranes on the side and back of the skull are starting to ossify. That means that the bone tissue is slowly growing over the area where the membranes once existed. Eventually, these bone plates will grow together forming the cranial cavity which protects the brain.

As the baby’s development is close to birth, you can see the bones of the skull still have gaps between them. These gaps, called fontanelles, allow room for the baby’s brain to grow, and also enable the head to be compressed during delivery.

The fontanelles will remain open until the end of the second year. And even though they’re commonly known as the baby’s soft spot, the fontanelles are actually about the thickness and strength of a piece of canvas. Which kind of makes them a soft, but tough, spot.

The bones of the skull won’t stop growing until a child reaches adulthood. That’s when the joints between the bones, called the sutures, will fuse together.

Now let’s go back once again and watch the second type of ossification when cartilage turns into bone. This time we’ll look at the hand. Most of the bones of the skeleton, like the arms, legs, ribs, fingers, and backbone, start off as cartilage.

We can get a good idea of how cartilage turns into bone by looking at this portion of the hand. Here’s what it looks like on the inside.

From the second month until the end of the third month, remarkable changes take place. Watch the middle of the cartilage: both the inside and the outside turn into bone, or ossifies.

This is how the bones will continue to grow until adulthood---from the middle of the bone outward. That way they can continue to increase in their length and width.

Regular workouts that include resistance training, such as weight lifting, cause the muscle fibers of skeletal muscle to increase in size and improve overall body tone.

Skeletal muscle is a well-organized body tissue, composed in a complex array of smaller and smaller structures. Let’s take a look at a cut-section to see how skeletal muscle is organized.

Each skeletal muscle is composed of many units called muscle fascicles. Notice that the fascicles are bound together by a type of connective tissue, which is called fascia. And from that fascicle, we can pull away smaller organizational units called muscle fibers.

And from that muscle fiber, we can pull away even smaller strands called myofibrils. Here you can see the myofibrils move as the muscle is contracting. It’s the interaction of these myofibrils as they slide and pull along side each other that gives skeletal muscle its functional ability to do work and move things.

Putting it all back together, myofibrils compose muscle fibers, that make-up muscle fascicles, which are grouped together within a skeletal muscle.

The skeletal system consists of approximately 206 bones, providing the body with structure and support. Let’s take a tour of various components that form the skeletal system.

Here’s the skull. It has 8 cranial bones that protect the brain. The facial skeleton has 14 bones that provide a framework for the eye sockets, jaws, and teeth. The facial bones provide the framework for the various structures of the face including the overlying muscles, fat and skin.

The vertebral column is composed of 24 individual vertebrae, along with two sets of fused bones called the sacrum and coccyx. In addition to providing support for the trunk of the body, the vertebral column protects the spinal cord. All together, there are 7 cervical, or neck vertebrae; 12 thoracic, or upper back, vertebrae; and 5 lumbar, or lower back, vertebrae.

The sacrum is composed of 5 fused bones, while the coccyx, or tailbone, is typically made up of 3 to 5 bones.

12 pairs of ribs form a protective cage for the heart, lungs, and other internal organs.

The first 7 ribs are called true ribs because they attach to the breastbone, or sternum. Ribs 8 through 12 are called false ribs, because they either attach indirectly, or, as in the case with ribs 11 and 12, float and don’t attach to the sternum at all.

Now let’s take a look at the pair of shoulder blades, or scapulae, and the collar bones, or clavicles. It is very important for the scapulae to be mobile, because they connect to the shoulder joint, which is the most movable joint in the body.

The bones of the upper limb include the humerus, which connects the shoulder with the elbow, the ulna, the radius, the wrist bones or carpals, the hand bones or metacarpals, and the finger bones or phalanges.

To complete our tour, let’s take a look at the pelvic girdle, knee, and foot.

The pelvic girdle is formed by a pair of hip bones. Each hip bone is comprised of 3 fused bones, the ilium, ischium, and pubis. The pelvic girdle connects with the femur or thigh bone at the hip joint. The femur is the longest bone in the body and is important for bearing the body’s weight while standing.

At the knee, the femur articulates with the tibia or shin bone. The fibula does not bear weight, but several muscles attach to it. The patella, or kneecap, is suspended within muscle tendons and glides over the femur and tibia when the knee bends.

And last, but certainly not the least, are the feet. The foot bones, which include the tarsals, metatarsals, and phalanges, are organized into a series of arches that allow the feet to support the body’s weight.

Let’s take a look at a few common skin conditions.

Moles are colored spots on the skin, formed by cells containing the dark pigment, melanin. While generally harmless, some moles can change shape and color, or start bleeding and require immediate evaluation.

Some birthmarks are simply moles that are present at birth, called a pigmented birthmark. Others result from the rapid growth of blood vessels in a localized area, called red birthmarks.

When a person becomes older, they may develop age spots. Age spots are patches of increased pigmentation on the skin’s surface, like freckles.

Warts are another very common type of skin disorder. Warts are benign, or non-cancerous, growths of skin caused by a virus.

The lungs are the primary respiratory organs and act as filters for the air the body breathes in.

Normally, they have a healthy pink color, as seen in this microscopic view of the alveoli.

Filtering smoke from the air breathed in can do damage to the lung tissue as seen on the right in this smoker’s lung. Over time, carbon molecules from inhaled smoke deposit in the lung tissue, giving it a blackened appearance.

Smoking can eventually lead to the formation of tumors and other serious lung diseases.

Smoking has also been linked to diseases that affect the cardiovascular system, such as atherosclerosis, which can lead to a heart attack or stroke.

The best advice is if you don’t smoke, don’t start. If you do smoke, it’s never too late to quit.

Many people snore when they sleep. Often, they may not even know they are snoring. Let's turn on the lights and see where the snoring is coming from.

Snoring occurs when the airway is partially blocked by the uvula. The lungs need to inhale harder to make up for the reduction in how much air is getting into the body. The snoring comes from the vibration of the soft palate at the back of the mouth and the uvula, which extends down from it and covers the airway.

Several things can cause someone to snore. For instance, it could come from drinking too much alcohol, nasal congestion, obesity, or enlarged tonsils and adenoids.

Snoring, by itself, is not necessarily dangerous. But some people that snore have such severe blockage of air that it keeps them from getting a good night's sleep. This condition is called "sleep apnea". It's common, but also dangerous if it's left untreated.

Sperm are produced, stored, and delivered by the male reproductive system. Here, you can see the parts of the male reproductive system -- the testes, urethra, vas deferens, prostate, seminal vesicle, and penis.

The testes contain coiled structures called seminiferous tubules, which are the sites of sperm production. A woman’s ovaries need only produce one egg per month, but a man’s seminiferous tubules produce over 12 billion sperm per month.

The structure on top of the seminiferous tubules is the epididymis. The immature sperm migrate there to mature, and then are stored there afterwards. This trip usually takes about 20 days.

Before intercourse, the penis fills with blood and becomes erect. Then, with sufficient stimulation, an ejaculatory process begins.

Here you can see the mature sperm traveling from the epididymis through the vas deferens, which is a narrow, muscular tube about 18 inches long. Its smooth muscle contractions propel the sperm forward. They arrive first at the ampulla, the widest part of the vas deferens, and then pass into the ejaculatory ducts. Here, a liquid secretion from the seminal vesicles mixes with the sperm. Seminal fluid contains fructose sugar, which the sperm use as fuel. It also has alkalines, which help to counteract the naturally acidic environment of the vagina and uterus so the sperm have a better chance for survival.

From there, the liquid mixture is propelled forward through the ejaculatory ducts toward the urethra, passing first through the prostate gland, where milky prostatic fluid is added, forming the substance we call semen. The prostatic fluid helps the sperm swim faster, which is important for getting to the egg cell.

Finally, about a teaspoon of semen is ejected out, or ejaculated, through the far end of the urethra at the end of the penis. From the time the sperm leave the man’s body, they have between 12 and 48 hours to find and fertilize the egg cell, assuming an egg is available. Most of the sperm won't make it. Of the 300 million sperm ejaculated, only about 200 or so will survive to reach the egg cell and only one of those will succeed in fertilizing it.

Sperm are produced and released by the male reproductive organs.

The testes are where sperm are produced. The testes are linked to the rest of the male reproductive organs by the vas deferens, which extends over the base of the pelvic bone or ilium, and wraps around to the ampulla, seminal vesicle, and prostate. The urethra then runs from the bladder through the penis.

Sperm production in the testes takes place in coiled structures called seminiferous tubules.

Along the top of each testicle is the epididymis. This is a cordlike structure where the sperm mature and are stored.

The release process starts when the penis fills with blood and becomes erect. Continuing to stimulate the penis will cause an ejaculation.

Mature sperm begin their journey by travelling from the epididymis to the vas deferens, which propels sperm forward with smooth muscle contractions.

The sperm arrive first at the ampulla just above the prostate gland. Here, secretions from the seminal vesicle located next to the ampulla are added.

Next, the seminal fluid is propelled forward through the ejaculatory ducts toward the urethra. As it passes the prostate gland, a milky fluid is added to make semen.

Finally, the semen is ejaculated from the penis through the urethra.

A stomach ulcer is caused by an imbalance between acid and pepsin secretion and the defenses of the stomach mucosal lining. Ulcers can be treated through dietary changes and medication.

A stroke can occur when an obstruction such as a blood clot travels from another part of the body and lodges inside an artery in the brain.

When an arterial wall becomes damaged, various types of emboli, or obstructions, can form. Emboli can be made up of various substances such as platelets, elements in the blood that help it clot, blood clots that form elsewhere and pass to the damaged area, cholesterol, or a combination of things.

For example, an embolism is formed in the carotid artery and breaks loose, traveling towards the brain where it will eventually lodge, blocking the blood the brain needs. The blocked artery deprives the brain of oxygen, which cause damage to the surrounding tissue. The result is a stroke.

A blood clot or embolus, can form and break off from the heart. The clot travels through the bloodstream where it can lodge in the artery of the brain, blocking the flow of oxygen-rich blood. The lack of oxygen results in damage, destruction, or even tissue death of the brain beyond the affected area.

The result is a stroke.

The skin uses sunlight to help manufacture vitamin D, which is important for normal bone formation. But there’s a downside. The sun's ultraviolet light can cause major damage to the skin. The outer layer of the skin has cells that contain the pigment melanin. Melanin protects skin from the sun's ultraviolet rays. These can burn the skin and reduce its elasticity, leading to premature aging.

People tan because sunlight causes the skin to produce more melanin and darken. The tan fades when new cells move to the surface and the tanned cells are sloughed off. Some sunlight can be good as long as you have proper protection from overexposure. But too much ultraviolet, or UV, exposure can cause sunburn. The UV rays penetrate outer skin layers and hit the deeper layers of the skin, where they can damage or kill skin cells.

People, especially those who don’t have much melanin and who sunburn easily, should protect themselves. You can protect yourself by covering sensitive areas, wearing sunblock, limiting total exposure time, and avoiding the sun between 10 am and 2 pm.

Frequent exposure to ultraviolet rays over many years is the chief cause of skin cancer. And skin cancer should not be taken lightly.

Check your skin regularly for suspicious growths or other skin changes. Early detection and treatment are key in the successful treatment of skin cancer.

This side view of the head highlights structures involved with swallowing. That includes the tongue, teeth, epiglottis, and esophagus.

The teeth grind and chop food into tiny pieces while the glands in the mouth moisten it with saliva.

Then the tongue pushes the moistened food, or bolus, to the back of the throat and down into the esophagus, which leads to the stomach.

Let's watch the swallowing process again.

First, the tongue pushes the food into the throat.

Next, the epiglottis, a small but important flap of tissue, folds over the voice box at the top of the windpipe. This keeps food from going down the wrong way.

Finally, the esophagus contracts and moves food toward the stomach.

Burp! Excuse me.

A body has between two and four million sweat glands lying deep in the skin. They are connected to the surface by coiled tubes called ducts. You perspire constantly, even without exercise. Sweat is a liquid made from 99% water and 1% salt and fat. Up to a quart of sweat evaporates each day.

When your body becomes overheated, you sweat more. The evaporation of sweat from your skin cools your body down.

When you're frightened or nervous (imagine being pinned under heavy weights) you also sweat more. Your palms and forehead begin to sweat. So do the soles of your feet and your armpits. These are sites where sweat glands are most abundant.

A heart beating at an abnormally fast rate of more than 100 beats per minute has the condition known as tachycardia. Tachycardia is a type of arrhythmia which is caused by an abnormality in the heart’s electrical system. Tachycardia can be treated surgically or with medication.

The tongue has about 10,000 taste buds. The taste buds are linked to the brain by nerve fibers. Food particles are detected by the taste buds, which send nerve signals to the brain.

Certain areas of the tongue are more sensitive to certain tastes, like bitter at the back of the tongue, sour along either side toward the back, sweet on the front surface, or salty around the front edge. Often, taste sensations are a mixture of these qualities.

Inside a pregnant woman’s uterus is an amniotic sac, which contains amniotic fluid and the growing fetus.

The amniotic fluid is important for several reasons -- it helps keep the baby warm, and because his body parts are growing so fast, the fluid provides lubrication that keeps them from growing together. In some cases, fingers and toes can become webbed as a result of not enough amniotic fluid circulating in the uterus.

The amniotic fluid also lets the baby move easily so he can exercise his muscles and strengthen his bones before he’s born.

In addition, it acts like a liquid shock absorber for the baby by distributing any force that may push on the mother’s uterus. Even sex won’t hurt the baby.

Amniotic fluid is 98% water and 2% salts and cells from the baby. Until the fetal kidneys started working during month four, amniotic fluid is made by the mother’s body. But after month four, the little guy started to make his contribution to the amniotic fluid by urinating in it.

You heard right. It may not sound appealing to us, but the urine in the amniotic sac is completely harmless to the baby.

The baby swallows amniotic fluid, which then passes through his digestive system, into his kidneys, and back out again to the amniotic sac as urine. In this way, he can practice using his digestive and urinary systems before he’s even born. In fact, doctors can tell by the amount of amniotic fluid whether the baby has difficulty with his swallowing reflex.

By the time he’s born, he will consume up to 13 ounces of amniotic fluid a day.

Here, we see a comparison of an artery affected with plaque in a non-smoker and smoker. The artery of the smoker is at greater risk for developing complications since smoking constrict arteries, predisposing them to clotting by altering platelet function and coagulability of blood. The result is blockage of the artery.

Twin to Twin Transfusion Syndrome, or TTTS, is a disease of the placenta, not the babies.

It affects twins or other multiples that share a single placenta with blood vessels that go from one baby to the other. In about 15% of cases, a shared vessel causes an unequal exchange of blood.

Here, blood from the smaller "donor" twin is transferred to the larger "recipient" twin. The twin that receives the blood is at risk for heart failure. Getting too much blood forces its heart to work harder. The other twin is at risk for loss of blood.

Laser surgery may correct the problem. A laser is endoscopically inserted into the womb. It then burns and seals the interconnecting blood vessels and restores the normal flow of blood.

Following treatment, the babies are regularly monitored.

Food enters the stomach from the esophagus. There, it's broken down by the acid and various enzymes the stomach produces so its nutrients can be absorbed in the small intestine. The inside wall of the stomach is protected from the acid and enzymes by a mucous lining.

Ulcers come about as a result of an imbalance between the stomach's digestive juices and the factors that protect its lining.

Symptoms can include bleeding. And on rare occasions, an ulcer may completely erode the stomach wall.

The bacteria Helicobacter pylori is a major cause of ulcers. Treatment typically includes medications to suppress the stomach acid and antibiotics to eradicate the infection.

Ultrasound is one of the most useful procedures for monitoring a baby's prenatal development. With ultrasound, doctors can check for defects of the head, spine, chest, and limbs; diagnose serious conditions like placenta previa or breech birth; and check to see whether the mother will have twins or triplets.

Ultrasound can be used anytime during pregnancy from the fifth week until delivery. It uses inaudible sound waves to "see" the baby inside the uterus. These sound waves bounce off solid structures in the body and are transformed into an image on a screen.

Here's how ultrasound works. Pretend this tennis ball is an organ in the body. This piece of glass represents the ultrasound image. Like this piece of glass, an ultrasound image is actually flat and two-dimensional.

If we could pass this tennis ball through the glass, the ultrasound image would show wherever the two are in contact. Let's watch the same thing on an ultrasound.

The white ring is the reflected image of the outer part of the tennis ball. Like many organs in the body, the tennis ball is solid on the outside, and hollow on the inside. Solid structures, like bones and muscles, reflect sound waves that show up as light gray or white images.

Soft or hollow areas like the chambers of the heart don't reflect sound waves. So they show up as dark or black areas.

In an actual ultrasound of a baby in the uterus, the solid structures in the baby’s body are transmitted back to the monitor as white or gray images. As the baby moves back and forth, the monitor shows the outline of his head. The eyes show as dark spots in the head. The region of the brain and the heart are also shown.

Remember, ultrasound only shows a flat image of the baby. A superimposed illustration of the fetus shows how the fetus actually looks in the uterus.

Ultrasound is still one of the best methods for physicians to visually diagnose major physical defects in the growing baby.

Even though there are no known risks for ultrasound at present, it is highly recommended that pregnant women consult their physician before undergoing this procedure.

The urinary system has four main components: the kidneys, ureters, urinary bladder, and urethra. Urine, a liquid waste product, is formed in the kidneys. From there it moves through the ureters and into the bladder, where it is stored. When the bladder gets full, urine is emptied from the body through the urethra in a process called urination.

The creation of urine is far more complex than you might think. The kidneys filter waste from the blood that passes through them, and reabsorb substances that the body requires, even though those requirements may change from moment to moment.

The outer portion of each of the kidneys is the cortex, while the inner portion is called the medulla.

Each of the kidneys is composed of approximately one million subunits called nephrons. Each nephron consists of a microscopic ball of blood vessels called a glomerulus, which is connected to a twisting length of tube called the renal tubule. Because the blood vessels in the glomeruli are porous, they act as filters, removing most of the water, salt, and waste from the blood that passes through them.

As filters, the glomeruli have physical properties that prevent large cells, like red blood cells, from passing into the renal tubules. On the other hand, smaller particles, like sugar and salt, can pass easily through the glomerulus.

It is in the renal tubule that waste products are passed into the urine. Substances the body needs, like water and salt, are reabsorbed back into the bloodstream at the same time.

The path of urine formation, reabsorption, and excretion begins at the glomerulus, continues through the renal tubules, and proceeds to the ureter.

For the most part, urine moves from the outer cortex of the kidneys to the inner medullary region. Urine then proceeds down the ureters and into the bladder.

Here you can see a cut-section of the bladder. The unique, expandable cells in the wall of the bladder stretch and become thinner as it fills. Finally, urine is excreted through the urethra.

Ouch!

Vaccines help to give the body immunity from infections. Different vaccines work in different ways. Some vaccines inject fragments of a virus or bacteria called antigens into the body. Once in the blood, these antigens circulate among the blood cells, which include red blood cells and white blood cells. White blood cells, such as B and T cells, help fend off foreign invaders.

When antigens invade tissue, they attract macrophages. These are scavenger cells that engulf the antigens. The macrophages then signal to T-cells that the antigens are invading. Killer T cells gather and attack the antigens. Then suppressor T cells stop the attack.

After the vaccination, B-cells make defensive antibodies against the antigen. These antibodies help the cells remember this particular antigen, so that they can fight it off if the body is infected again.

When the cervix dilates to 10 centimeters, the pushing and delivery phase of childbirth begins.

During this phase, the baby starts the journey down the birth canal. As the baby’s head rotates, it may become distorted while slowly coming down the narrow opening. The baby’s skull bones have gaps called fontanelles that allow the head to elongate and fit within the birth canal.

As the baby’s head is delivered, it will naturally turn to one side. The baby’s head and shoulders are supported and the rest of the baby’s body generally comes out fairly quickly.

Often when parents see their newborn child, they realize that the pain and waiting were all worth the effort.

Welcome to the world, little guy.

A vasectomy is a procedure to cause permanent sterility in a man by preventing the transport of sperm out of the testes.

A small incision is made in the scrotum, which is the skin containing the testes, and each vas deferens is tied off and cut apart preventing sperm from being released within the ejaculate.

The small skin incision is stitched closed.

A vasectomy does not affect a man's sexual function.

As a person inhales, air and scent molecules move past the smell receptors in the nose. In turn, the smell receptors relay a signal to the brain. Smells can trigger memories and emotional responses.