How the heart works.

The normal heart is a strong, muscular pump a little larger than a fist. It pumps blood continously through the circulatory system. Each day, the average heart expands and contracts 100,000 times and pumps about 2,000 gallons of blood. In a 70-year lifetime, an average human hearts beats more than 2,500,000,000 times.

The heart has four chambers. The upper two are the atria; the lower two, the ventricles. Blood is pumped through them, aided by four valves that open and close to allow blood to flow in only direction when the heart contracts.

The four heart valves are the tricuspid, located between the right atrium and right ventricle; the pulmonary, between the right ventricle and the pulmonary artery; the mitral, between the left atrium and left ventricle; and the aortic, between the left ventricle and the aorta. Each valve has a set of flaps (also called leaflets or cusps). The mitral valve has two flaps; the others, three. Under normal conditions, the valves permit blood to flow in just one direction. Blood flow occurs only when there is a difference in pressure across the valves that causes them to open.

The heart pumps blood to the lungs and to all the body's tissues by a sequences of highly organized contractions of its four chambers:

The right atrium receives blood from the veins. This blood carries little oxygen and lots of carbon dioxide, since it is returning from the body's tissues, where much of the oxygen was removed and the carbon dioxide added. Venous blood is darker in color than arterial blood because of the difference in dissolved gases.

While he heart is ralaxed, venous blood flows through the open tricuspid valved to fill the right ventricle. An electrical signal starts the heartbeat by causing the atria to contract. This contraction "tops off" the filling of the ventricle. Shortly after the atrium contracts, the right ventricle contracts. As this occurs, the tricuspid valve closes and the partially deoxygenated blood is...