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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.
Structure and Function
Blood from the body returns to the right atrium through two large veins, the superior and inferior venae cavae; in addition the blood that has supplied the heart muscle is drained directly into the right atrium through the coronary sinus. Return of venous blood to the right atrium takes place during the entire heart cycle of contraction and relaxation, and to the right ventricle only during the relaxation part of the cycle, called diastole, when both right heart cavities constitute a common chamber; near the end of diastole, contraction of the right atrium completes the filling of the right ventricle with blood. Rhythmic contractions of the right ventricle expel the blood through the pulmonary arteries into the capillaries of the lung, where the blood receives oxygen. The lung capillaries then empty into the pulmonary veins, which in turn empty into the left atrium. Pulmonary venous return to the left atrium and left ventricle proceeds simultaneously in the same manner as the venous return to the right heart cavities. Contraction of the left ventricle rhythmically propels the blood into the aorta and from there to all arteries of the body, including the coronary arteries which supply the heart muscle.
The blood forced from the ventricles during systole, or contraction, is prevented from returning during diastole by valves at the openings of the aortic and pulmonary arteries. These valves consist of three semilunar (half-moon-shaped) flaps of membrane, which are curved in the direction of blood flow and which open readily on pressure in that direction; when the original pressure subsides, back pressure forces the edges of the flaps together. The tricuspid valve, situated between the right atrium and ventricle, is composed of three triangular flaps of membrane, and the bicuspid or mitral valve, between the left atrium and ventricle, has two such flaps. The bases of the flaps in both these valves are attached in a groove at the junction of the atrium and ventricle; the free edge is anchored by tendinous cords, known as the chordae tendinae, to the muscles of the heart wall. The flaps remain open until the ventricle fills with blood. When the ventricle begins to contract, the valve is closed by pressure. The chordae tendinae prevent inversion of the flaps during this period of systolic pressure.
The rate of heartbeat is controlled by the autonomic nervous system, being accelerated by the sympathetic system and depressed by the parasympathetic system. Nervous impulses originate rhythmically in a special nervous node, the sinus, or sinoatrial node, located in the right atrium near the superior vena cava. The impulses travel along a slender bundle of neuromuscular fibres, called the bundle of His, or the atrioventricular bundle, which, after a relay at the atrioventricular node, branches into two bundles, one for each of the ventricles. Through these specialized fibres the nervous impulse stimulates, at rapid speed and in a known orderly sequence, contraction of the heart-muscle fibres.
Although the nerves and certain drugs influence the rate of the heartbeat-as do certain hormone-like chemicals found in the 1980s to be secreted by the heart itself-heart muscle is inherently contractile and will continue beating at a regular rate even when all connection with the nervous system is severed.
In the embryo the heart develops from the fusion of the two ventral aortas, forming a single pulsating organ. Separation into right and left heart takes place later with the formation of an interauricular and an interventricular septum (partition). The separation is not completed, however, until the lungs begin their function at birth. Before birth the blood is oxygenated in the placenta and returns to the right auricle through the inferior vena cava; it is then directed by the Eustachian valve through the foramen ovale, a persistent opening in the interauricular septum. After birth the Eustachian valve degenerates to a rudiment and the foramen ovale normally closes, but the opening in the interauricular septum may persist in varying degree even in the adult in about one-fifth of the population.
The heart produces two sounds in each cycle of the beat. The first is rather dull and is caused by vibration of the auriculoventricular valves and by the contraction of the ventricular muscle fibres; the second is sharp and is caused by the sudden closing of the aortic and pulmonary valves. In cardiac disease these regular sounds may be replaced or accompanied by murmurs caused by turbulent blood rushing through abnormal valves or openings; detection of such murmurs is of great importance in diagnosis.
Heart Diseases
Disorders of the heart kill more people in developed nations 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 foetal 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 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, toxic goitre, 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; there is always the risk of 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 diseases. 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 beta blockers (for example, propranolol) and nitrates, which reduce the load on the heart. 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.
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 several 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. However, coronary heart disease remains the leading cause of mortality in Britain.
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 the spontaneous contraction of an apparently healthy coronary artery (vasospasm) which 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-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 fibres 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. 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 pulmonary heart disease, which is usually 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 far 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.
Diagnosis
The electrocardiograph, an instrument for recording the electrical currents produced by the heart muscle during various phases of contraction, is an important diagnostic tool. The efficiency of the heart as a pump may be measured accurately by the use of cardiac catheterization. In this technique a tube is introduced, through a vein or an artery or both, into the right, left, or both heart cavities, the pulmonary artery, and the aorta. This process permits determination of the rate of blood flow and recording of blood pressure in intracardiac and large vessels. This technique makes it possible to detect abnormal communications between right and left heart cavities. In another diagnostic technique called angiocardiography, or cinefluoroscopy, photographic recordings are obtained of the heart cavities and of the pathways and contours of the pulmonary vessels and the aorta with its branches; the technique involves injecting a substance opaque to X-rays into a vein. Even more accurate delineation of areas of reduced blood flow in the heart is provided by a new technique that visualizes the flow of a radioactive isotope of the element thallium into heart muscle. A computerized camera records the extent of thallium penetration during the systole-diastole cycle of the heart, precisely showing small areas of tissue damage. Yet another technique that is now being used is ultrasound-ultrasonic imaging.
Heart Transplants
In 1967 a human heart from one person was transplanted into the body of another by the South African surgeon Christiaan Barnard. Many surgeons have since adopted the procedure. The major problem at first was the body's natural tendency to reject tissues from another individual. By the early 1980s, however, due to the use of immunosuppressive drugs, particularly cyclosporine, many more cardiac transplant recipients were living beyond one year. By the 1990s the operation had become more commonplace in developed nations, such as the US and Britain, with many patients living for five to ten years following a heart transplant.
Artificial Hearts
Artificial hearts have been under development since the 1950s. In 1966 a booster pump was successfully implanted for the first time as a temporary measure; at least one such pump continued to work for several years. In 1969 the first completely artificial heart was implanted in a human, again on a temporary basis. The first permanent artificial heart, designed by 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 tended to suffer strokes and related problems.
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