YOU AND YOUR HEART: HOW DOES THE HEART WORK?

The normal adult heart is a strong, hard working muscular pump, about the size of a fist. It beats continuously 50-150 times each minute, driving blood around the body and carrying to every living cell the oxygen, nutrients and other chemical substances needed to maintain life. In most people the resting pulse rate is between 60 and 70 beats per minute.

The heart is divided into four chambers. Blood is pumped through these chambers, aided by a series of valves which open and close to allow onward progress of the blood. Dark blue venous blood, low in oxygen, returns to the heart through veins and enters an upper chamber (the right atrium). This low-pressure chamber then empties into the lower chamber (right ventricle) which drives the blood under pressure through the pulmonary artery to the lungs. There the blue blood receives oxygen and loses carbon dioxide. After blood is oxygenated, its appearance is bright red. This blood returns from the pulmonary vein to the left atrium, and enters the left ventricle after passing through the mitral valve. The left ventricle pumps the red oxygenated blood out through the aortic valve into the aorta where it is distributed through divided arteries into the body's general circulation.

The heart muscle (myocardium), like any other tissue, needs oxygen and nutrients. Its blood supply is provided by the two arteries which branch off from the aorta and surround the heart like the rim of a crown on a king's head. Anything which interferes seriously with the normal blood flow in the coronary arteries impairs the heart's pumping action.

By far the most important cause of reduced coronary blood flow is the narrowing of the coronary arteries due to arteriosclerosis. Arteriosclerosis — thickening and hardening of the arterial wall — is caused by a build-up of fatty deposits, such as cholesterol, on the artery wall. Thrombosis (formation of a blood clot) sometimes occurs on a fatty patch and completely blocks the artery.

Such pathological changes in the coronary arteries give rise to three forms of coronary heart disease: sudden death (usually the result of severe narrowing, with or without a clot); an 'acute heart attack (myocardial infarction), which scars the heart muscle; or angina pectoris, characterised by a severe pain in the chest, sometimes spreading up into the neck or down the arms. Angina, the heart's 'cry for oxygen', is generally brought on by exertion, particularly in cold weather or after a heavy meal. It is usually relieved by resting.

By the age of 40, nearly everyone has some narrowing of the coronary arteries, but, fortunately, it is usually not severe enough to interfere with the heart's blood supply. One important effect of regular exercise is that it improves the heart's pumping efficiency. The heart is able to take on a given work-load with less expenditure of effort. There is less oxygen demand.

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