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.