The Circulatory System

In its ceaseless work, the heart contracts more than 100,000 times a day to drive blood through about 60,000 miles (96,000 kilometers) of vessels to nourish each of the trillions of cells in the body. Each contraction of the heart forces about 2.5 ounces (74 milliliters) of blood into the bloodstream. This adds up to about 10 pints (4.7 liters) of blood every minute. An average heart will pump about 1,800 gallons (6,800 liters) of blood each day. With exercise, that amount may increase as much as six times. In an average lifetime, the heart will pump about 100 million gallons (380 million liters) of blood.

The heart is divided into four chambers. The upper chambers are the atria (singular atrium). The lower chambers are the ventricles. The wall that divides the right and left sides of the heart is the septum. The atria are thin-walled holding chambers for blood that returns to the heart from the body. The ventricles are muscular chambers that contract rhythmically to propel blood through the body.

Movement of blood between chambers and in and out of the heart is controlled by valves that allow movement in only one direction. Between the atria and ventricles are atrioventricular (AV) valves. Between the ventricles and the major arteries into which they pump blood are semilunar (SL) valves. The "lub-dup" sounds that a physician hears through a stethoscope occur when the heart valves close. The AV valves produce the "lub" sound upon closing, while the SL valves cause the "dup" sound.

Blood carrying no oxygen (oxygen-depleted) returns to the right atrium of the heart through the vena cava, a major vein. It then passes through the tricuspid or right AV valve into the right ventricle. The tricuspid valve is so named because it has three cusps or flaps that open and close to control the flow of blood. When the right ventricle contracts, blood is forced from the heart into the pulmonary artery through the pulmonary SL valve.

The pulmonary artery is the only artery in...