The human heart is a muscular pump. Its function is to pump blood containing oxygen, glucose, protein, fat, and salts to every organ, tissue, and living cell of the body. The heart is divided into four chambers. The upper chambers are called the right and left atrium, and the lower chambers are called the right and left ventricles. Blood from all parts of the body drains into veins that empty into the right atrium. Blood passes from the right atrium through the tricuspid valve and reaches the right ventricle. During contraction of the right ventricle, blood is pushed into the lungs where it gives off CO2, takes up oxygen, and returns via the pulmonary veins to the left atrium (Fig. 4).
During relaxation or diastole of the left ventricle, blood passes from the left atrium through the mitral valve (labeled bicuspid in Figs. 3 and 5) to reach the left ventricle. When the left ventricle contracts simultaneously with the right, about 70 ml of blood is ejected with each heartbeat through the aortic valve into the aorta. The blood is then circulated through the branches of the aorta that form the arterial system supplying blood to organs and tissues of the body.
If the heart beats 70 times per minute, it produces an output from the heart of approximately 5 L of blood per minute. This is called the cardiac output. Each 70 ml of blood is propelled through approximately 100,000 km of blood vessels. The heart beats about 2.5 billion times during an average lifespan, pumping more than 227 million L of blood. Fortunately, the heart muscle is one of the strongest in the body. It can maintain efficient pump¬ing and life for more than 100 years provided the coronary arteries that feed the muscle with blood do not become blocked by hardened plaques or a blood clot. Unfortu¬nately, the coronary arteries are very narrow and are easily obstructed by plaques containing cholesterol, calcium, and other constituents (see the chapter Atherosclerosis/ Atherothrombosis).

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