Heart, in anatomy, the hollow muscular organ that receives blood from the veins and propels it into and through the arteries. The heart of a human is about the size of a closed fist. It is situated behind the lower part of the breastbone, extending more to the left of the midline than to the right. It is roughly conical in shape, with the base directed upward and to the right and slightly backward; the apex touches the chest wall between the fifth and sixth ribs. The heart is held in place principally by its attachment to the great arteries and veins, and by its confinement in the pericardium, a double-walled sac with one layer enveloping the heart and the other attached to the breastbone, the diaphragm, and the membranes of the thorax.
Within the adult heart are two parallel independent systems, each consisting of an auricle, or atrium, and a ventricle; from their anatomical positions these systems are often designated the right heart and the left heart.

Heart Diseases

Disorders of the heart kill more Americans than any other disease. They can arise from congenital defects, infection, narrowing of the coronary arteries, high blood pressure, or disturbances of heart rhythm.
Congenital heart defects include persistence of fetal connections between the arterial and venous circulations, such as the ductus arteriosus, a vessel normally connecting the pulmonary artery and the aorta only until birth. Other important developmental anomalies involve the partition separating the four cardiac cavities and the large vessels issuing from them. In newborn "blue babies," the pulmonary artery is narrowed and the ventricles are connected by an abnormal opening; in this cyanotic condition, the skin has a bluish tinge because the blood receives an insufficient amount of oxygen. Formerly the expectation of life for such infants was extremely limited; with the advent of early diagnosis and improved techniques of hypothermia, surgery is often possible in the first week of life and the outlook for these infants is greatly improved.
Rheumatic heart disease was formerly one of the most serious forms of heart disease of childhood and adolescence, involving damage to the entire heart and its membranes. It usually followed attacks of rheumatic fever. Widespread use of antibiotics effective against the streptococcal bacterium that causes rheumatic fever has greatly reduced the incidence of this condition.
Myocarditis is inflammation or degeneration of the heart muscle. Although it is often caused by various diseases such as syphilis, goiter, endocarditis, or hypertension, myocarditis may appear as a primary disease in adults or as a degenerative disease of old age. It may be associated with dilation (enlargement due to weakness of the heart muscle) or with hypertrophy (overgrowth of the muscle tissue).
The major form of heart disease in Western countries is atherosclerosis. In this condition fatty deposits called plaque, composed of cholesterol and fats, build up on the inner wall of the coronary arteries. Gradual narrowing of the arteries throughout life restricts the blood flow to the heart muscles. Symptoms of this restricted blood flow can include shortness of breath, especially during exercise, and a tightening pain in the chest called angina pectoris. The plaque may become large enough to completely obstruct the coronary artery, causing a sudden decrease in oxygen supply to the heart. Obstruction, also called occlusion, can occur when part of the plaque breaks away and lodges farther along in the artery, a process called thrombosis. These events are the major causes of heart attack, or myocardial infarction, which is often fatal. Persons who survive a heart attack must undergo extensive rehabilitation and risk a recurrence.
Development of fatty plaque is due partly to excessive intake of cholesterol and animal fats in the diet. A sedentary life-style is thought to promote atherosclerosis, and evidence suggests that physical exercise may help prevent heart disease. A striving, perfectionist temperament referred to as Type A personality has also been associated with increased risk of heart attacks, as has cigarette smoking. The occurrence of the heart attack itself is much more likely in persons who have high blood pressure. The actual event precipitating the attack may involve products secreted by platelets in the blood. This has led to clinical studies testing whether persons who have had a heart attack will be protected from a second infarction if they take drugs that block the action of platelets.
Many persons having severe angina because of atherosclerotic disease can be treated with drugs, such as propranolol and nitrates, which enable the heart to work more efficiently. Those who do not obtain relief with pharmacologic means can often be treated by a form of surgery called coronary bypass. In this procedure, which became established in the 1970s, a section of vein from the leg is sewn into the blocked coronary artery to form a bridge around the atherosclerotic region. In most recipients the operation relieves the pain of angina and in many persons it prevents a fatal heart attack. By 1986 more than 225,000 patients were undergoing these procedures each year in the U.S.
A second surgical procedure that was developed during the 1970s to treat atherosclerotic heart disease is balloon catheterization, technically called percutaneous transluminal coronary angioplasty. In this operation a wire with a balloon on the tip is inserted into an artery in the leg and threaded through the aorta into the coronary artery. When the balloon reaches the atherosclerotic area, it is inflated. The plaque is compressed and normal blood flow is reestablished. It is estimated that about one in six coronary bypass operations can be replaced by this less dangerous procedure.
During the 1970s and early 1980s it became apparent that a dramatic drop was occurring in mortality from atherosclerotic heart disease in developed countries. Although no definitive explanation for this decline has been given, public health officials have attributed it to widespread detection and treatment of high blood pressure and a decrease in the amount of animal fat in the average Western diet.
Some persons who die of apparent heart attack exhibit no evidence of severe atherosclerosis. Research has shown that a decrease in blood flow to the heart can also be from vasospasm, the spontaneous contraction of an apparently healthy coronary artery. The existence of this phenomenon was documented in 1978 by Italian cardiologists. Vasospasm may contribute to some heart attacks brought on by atherosclerosis.
The immediate cause of death in many heart attacks, whether atherosclerosis is present or not, is ventricular fibrillation, also called cardiac arrest. This is a rapid ineffective beating of the ventricles. Normal heart rhythm can often be restored by a massive electric shock to the chest, a finding that has led to emergency rescue teams in many cities being trained in this technique.
Minor variations in the heart rhythm usually have little pathological significance. The heart rate responds to the demands of the body over such a wide range that variations are generally within normal limits. Severe defects, however, in the sinoatrial node or in the fibers that transmit impulses to the heart muscle can cause dizziness, faintness, and eventually death. The most serious of these conditions is called complete heart block. It can be corrected by insertion of an artificial pacemaker, a device that gives timed electric shocks to make the heart muscle contract in a regular pattern. More than 80,000 pacemakers are permanently implanted each year in the U.S. Most other arrhythmias are not dangerous except in persons with underlying heart disease. In these patients, especially those who have already had a heart attack, arrhythmias are treated with propranolol, lidocaine, and disopyramide.
Often found among older persons is cor pulmonale, or pulmonary heart disease, which usually is the result of a lung ailment, such as emphysema, or a disease affecting circulation to the lungs, such as arteriosclerosis of the pulmonary artery. Another condition found in older persons is congestive heart failure, in which the ventricles pump much less efficiently. The muscular walls of the ventricles enlarge with the effort to propel more of the blood into the circulation, giving rise to the large, floppy hearts characteristic of this syndrome. Persons with this ailment have a reduced capacity for exercise. Their condition can often be improved with one of the derivatives of digitalis, which increases the pumping efficiency of the heart.

Heart Transplants

In 1967 a human heart from one person was transplanted into the body of another. South African surgeon Christiaan Barnard performed the first such transplant; many surgeons adopted the procedure. Because most patients were dying soon after a transplant, however, the number of operations dropped from 100 in 1968 to 18 in 1970. The major problem was the body's natural tendency to reject tissues from another individual. In Palo Alto, California, the surgeon Norman Shumway persisted in working on this problem, and in the early 1980s he and his colleagues could report that more than half of all cardiac transplant recipients were living beyond one year. Pharmaceutical aids such as cyclosporine are helping to reduce the immune reaction. In 1984 the transplantation of a baboon heart into a female infant was also attempted, in a controversial California operation, but the heart was eventually rejected.

Artificial Hearts

Artificial hearts have been under development since the 1950s. In 1966 Dr. Michael Ellis DeBakey successfully implanted a booster pump for the first time as a temporary measure; at least one such pump continued to work for several years. In 1969 Dr. Denton Cooley implanted the first completely artificial heart in a human, again on a temporary basis. The first permanent artificial heart, designed by Dr. Robert Jarvik, was implanted in 1982 in a patient who lived for three months thereafter. A number of patients have received Jarvik and other artificial hearts since that time, but surviving recipients thus far have tended to suffer strokes and related problems.