Heart Health Article

Heart

The organ that pumps the blood for circulating throughout the human body.

The human heart is a pulsating, four-chambered organ that circulates blood through the body. Though the heart appears to be a simple organ, it requires a complex series of nerve stimulations, valve openings, and muscle contractions to adequately achieve its purpose.

The human heart on the average weighs about 10.5 oz (300 g). Cone-shaped and about the size of a closed fist, it lies in the mid-thorax, under the breastbone (sternum). Nestled between the lungs, the heart is covered by a fibrous sac called the pericardium. This important organ is protected within a bony cage formed by the ribs, sternum, and spine.

In its ceaseless work, the heart contracts some 100,000 times a day to drive blood through about 60,000 miles (96,000 km) of vessels to nourish each of the trillions of cells in the body. Each contraction forces about 2.5 oz (0.075 liter) of blood into the circulation, which adds up to about 10 pints (4.7 liters) of blood every minute. On the average, the heart will pump about 2,500 gallons (9,475 liters) of blood in a day, and that may go up to as much as 5,000 gallons (18,950 liters) with exertion. In a lifetime the heart will pump about 100 million gallons of blood.

Oxygen-depleted blood returns to the right atrium—or holding chamber—of the heart, from which it passes through the tricuspid or right atrioventricular valve into the lower right chamber, the 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 semilunar valve. The thin, fibrous flaps of the semilunar valve have strong fibers attached to them. These fibers are also attached to the wall of the ventricle. These cords prevent the valve from ballooning up into the atrium when the ventricle contracts.

The blood flows through the pulmonary artery, the only artery in the body that carries unoxygenated blood into the lungs, where it loses carbon dioxide and other impurities and picks up oxygen. The freshly oxygenated blood then returns to the left side of the heart through the four pulmonary veins, which empty into the left atrium. The contents of the atrium then pass through the left atrioventricular, or mitral, valve into the left ventricle.

The left ventricle has the hardest task of any chamber in the heart. It must force blood from the heart into the body and head. For that purpose, it has a much thicker wall, approximately three times thicker than the right ventricle wall. When the left ventricle contracts, blood passes through the aortic semilunar valve into the largest artery in the body (the aorta) to be carried and distributed to every area of the body.

The wall that divides the right and left sides of the heart is the septum. Occasionally, the septum is imperfectly formed, and may have an opening in it that allows blood from the right and left sides to mix. If this septal defect is minor, there are usually no serious medical consequences. However, a larger opening that allows too much blood to mix, thus preventing the left ventricle from applying sufficient pressure, will require surgery. A patch can be placed over the opening to seal it and ensure normal function.

The heart muscle is unique in that it is not under voluntary control, and must work incessantly. The heart muscle, or myocardium, requires considerable nourishment, and the arteries that feed the myocardium are the first to branch off from the aorta. These are the coronary arteries that pass down and over the heart to provide a copious and uninterrupted blood supply to it. Any interruption of blood flow through these arteries can trigger a heart attack.

This coordinated pattern of chambers filling and emptying in sequence is controlled by a system of fibers providing the electrical stimulus to trigger contraction of the heart muscle. The initial stimulus is provided by a small node of specialized tissue in the upper area of the right atrium. This is called the sinoatrial node, or SA node. Under the influence of this node, the heart beats at a sinus rhythm, which is normal. The SA node fires an electrical impulse that spreads across the atria, causing them to contract. The signal also reaches another node, the atrioventricular or AV node, lying near the bottom of the right atrium just above the ventricle. This node receives the electrical signal from the SA node, and sends out its own electrical impulse. The AV impulse travels down a specialized train of fibers into the ventricular muscle. This causes the ventricles to contract and expel their contents. Thus, the contraction of the atria is made to occur slightly before the contraction of the ventricles. The heart's electrical activity can be measured by a device called the electrocardiograph (ECG or often abbreviated EKG).

Variations in the electrical system can lead to serious, even dangerous, consequences. When that occurs, an artificial electrical stimulator called a pacemaker must be implanted to take over the regulation of the heartbeat. A small pacemaker can be implanted under the skin near the shoulder, and long wires from it are fed into the heart and implanted in the heart muscle. The pacemaker can be regulated for the number of heartbeats it will stimulate per minute. Newer pacemakers can detect the need for increased heart rate when the individual is under exertion or stress, and will respond.

Heart disease is the leading cause of death among older Americans. Heart attacks—blockage of a coronary artery—and atherosclerosis or "hardening of the arteries"—deposits of fat on coronary artery walls—are two of the most common conditions that lead to impaired health and fatalities. Both are preventable in most cases, particularly by closely monitoring the health of children and adolescents and by teaching young people to care for their health.

Smoking, high blood pressure, obesity, high cholesterol levels and diabetes are all risk factors in heart disease. Young people who eliminate bad habits such as smoking, and try to eat nutritious, low-fat diets, and exercise regularly have a greater chance of maintaining healthy hearts throughout their adult years. Many physicians advocate periodic checks of blood cholesterol levels beginning at the age of two, particularly for children with a family history of heart disease. Up to 15% of children in the United States are believed to have high cholesterol levels which, left untreated, can lead to heart disease. All teens are encouraged to have their blood pressure checked periodically, particularly if they are overweight and have a family history of high blood pressure, diabetes, or stroke.

"Hypertrophie cardiomyopathy," an extreme thickening of the walls of the heart's chambers, occurs mainly in young athletes. It is a rare condition that garners much publicity. Athletes are usually stricken during intense physical exercise, sometimes during competitions. Fainting and complete heart failure can result. Young athletes are encouraged to get complete physicals that include chest X-rays, which can detect the otherwise silent condition.

Less than one percent of infants are born with congenital heart defects or other heart-related problems. While they are difficult to predict, some risk factors have been identified, including: congenital heart disease in either or both parents, sibling or relative; maternal diabetes; alcohol or drug abuse by the mother during pregnancy; rubella, toxoplasmosis, or HIV infection of the mother; exposure of the mother to certain medications during pregnancy.

Doctors are able to detect many heart problems soon after birth and often before birth. Some can be treated prenatally. Common problems include structural defects that hinder the normal circulatory pattern of the blood, connection problems between the main arteries and veins and the heart itself, malformed or missing heart valves, and other defects that block the blood flow through the vessels.