Home > Encyclopedia of Nursing & Allied Health > Heart

Definition

The heart is a muscular organ of the cardiovascular system that contracts to cause movement of the blood throughout the body.

Description

The heart is approximately fist-sized and located in the chest between the two lungs and behind the ribs and breastbone (sternum). It rests at a slight tilt from vertical, which makes it appear to be on the left side of the body. The walls of the heart are made up of three layers of tissue: epicardium, myocardium, and endocardium. The epicardium is a thin layer on the outer surface of the heart. The myocardium is the muscular layer, made up of cardiac muscle that contracts to do the work of the heart moving the blood. The endocardium is the smooth inner lining of the heart.

The entire structure of the heart is enclosed in a fibrous sac called the pericardium. A small amount of liquid is normally found in the space between the heart and the pericardium, which helps reduce the friction between the epicardial and pericardial membranes.

The heart is divided by a central wall (or septum) into its right and left sides. Each of these sides contains a smaller, upper chamber known as an atrium, and a lower, larger chamber known as a ventricle. The atria and ventricles are separated by a valve made of flaps of tissue that prevent blood flow in the wrong direction. The valve on the left side of the heart is the mitral (or bicupsid) valve, which has two flaps. The right atria and ventricle are separated by the tricupsid valve, which has three flaps.

There are five great vessels branching off from the heart that are responsible for carrying blood into or out of the organ. These five vessels are the aorta, the pulmonary artery and vein, and the superior and inferior venae cavae. The aorta is the main artery, carrying oxygenated blood from the heart out into the body. The pulmonary artery carries blood away from the heart to the lungs, and the pulmonary vein carries blood from the lungs to the heart. The superior and inferior venae cavae carry deoxygenated blood from the upper and lower parts of the body back to the heart.

Unidirectional valves separate two of the great vessels from the chambers of the heart. The pulmonic (or pulmonary valve) separates the right ventricle and the pulmonary artery. The aorta and left ventricle are separated by the aortic valve.

The coronary arteries are two vessels that divide off the aorta and branch out over the entire surface of the heart. These vessels bring oxygenated blood to the heart tissue itself.

Function

The heart functions as a strong, four-chambered muscular pump. It can move more than five quarts of blood through the body each minute, the equivalent of about 2,000 gallons per day. At a typical heart rate of 72 beats per minute, the heart contracts on average 100,000 times per day. This adds up to more than 2.5 billion beats in a 70-year lifetime.

One key to the functioning of the heart is the unique characteristics of its muscular tissue. Cardiac muscle differs from other muscles of the body in that its normal function is a rhythmic contraction, which is the basis for the tissue's ability to respond to the electrical impulses that govern the beating of the heart. The natural pacemaker of the heart, the sinoatrial (SA) node, is located in the right atrium. Cardiac muscle cells that naturally contract at the fastest rate when compared to the other cells of the heart surround this cluster of nerve cells. This area of the heart therefore has the ability to initiate the contraction by sending wavelike electrical signals throughout the organ.

First, the electrical signal causes the two atria to contract, when sends the blood from those chambers into the two ventricles. Then the signal passes down through a group of nerve cells known as the atrioventricular (AV) node. This nerve cluster is located near the center of the heart. The travel through this area slows down the signal so that it reaches the ventricles after the atria have finished their contraction. Then the ventricles contract, moving the blood out of the heart, and the cycle starts again. The heart's electrical activity can be measured using electrocardiography.

The physical functions of the full heartbeat is known as the cardiac cycle. The cycle can be divided into two phases: diastole and systole. Diastole occurs when the heart relaxes and the myocardial fibers lengthen. As the heart dilates, the cavities fill with blood. Diastole of the atria occurs slightly before the diastole of the ventricles.

Systole happens when the part of the heart is in contraction and the myocardial fibers shorten. Again, systole of the atria precedes systolic phase of the ventricles. Systole of the ventricles cause blood to surge out of the heart and into the aorta and pulmonary artery.

Over time, the cardiac cycle occurs as follows. It begins with the diastole of the atrium, where both the left and right atria relax and fill with blood. The right atria fills with deoxygenated blood from the superior and inferior venae cavae. The pulmonary artery fills the left atria with newly oxygenated blood from the lungs. The SA node signals the beginning of systole and the atrium contract, sending blood through the tricuspid and mitral valves into the right and left ventricles, respectively. During ventricle filling, the valves of the great vessels are closed so blood already pumped out of the heart does not leak back.

The electrical signal has now reached the ventricles and they contract, sending the deoxygenated blood of the right ventricle into the pulmonary artery to the lungs and the oxygenated blood of left ventricle into the aorta to the body. During contraction, the tricupsid and mitral valves close to prevent flow back into the atrium. This cycle is repeated continuously.

Role in human health

The heart is the centerpiece of the elaborate and extensive human cardiovascular system. Responsible for moving the blood throughout the body, this system transports the necessities for life—oxygen, nutrients, hormones, immune functions—to the cells. The system also transports wastes such as carbon dioxide away from the cells to the organs responsible for their elimination from the body. The heart is the driving force behind this essential

---

KEY TERMS

---

Diastole—Phase of the heartbeat where the ventricles relax and fill with blood.

Endocardium—The thin, innermost layer of the heart.

Epicardium—The outermost layer of the heart.

Myocardium—The middle, working layer of the heart containing the heart muscle cells.

Regurgitation—A defect of the heart valves that interferes with its ability to close completely, allowing blood to leak in the direction opposite of circulation.

Septum—A physical divider between chambers, found between the atria and the ventricles.

Stenosis—A stiffening of the heart valves, which narrows its opening and can interfere with function.

Systole—Phase of the heartbeat where the ventricles contract and force blood from the heart.

---

transport. Therefore it is not surprising that a healthy heart is necessary for a healthy body.

There are several risk factors for heart problems that can be controlled through preventative measures. These include lowering blood cholesterol levels with diet, exercise, or in extreme cases, medications. Lower cholesterol levels lower the probability of coronary artery disease (clogging of the arteries that bring oxygenated blood to the heart), a major cause of heart failure. Keeping blood pressure, blood sugar levels, and body weight within normal levels also greatly decreases the chance for heart problems. Other controllable risk factors include a sedentary lifestyle, stress and anger, and smoking.

Common diseases and disorders

According to 1999 estimates, approximately 58.8 million Americans have one or more types of heart disease, as it is most broadly defined. Although the cause and effect of various heart diseases can be examined in a particular patient, it is difficult to make generalizations about different diseases and disorders. Often one disease of the cardiovascular system will contribute or even cause another. In any case, the various symptoms of an unhealthy heart, whatever the cause, are quite common.

Several diseases of the heart are related to athero-sclerosis, the accumulation of cholesterol in the arteries. When this problem occurs in the coronary arteries, it is known as coronary artery disease. Three conditions which can follow from the loss of blood flow to the heart due to the clogged arteries are angina pectoris, a severe chest pain, myocardial infarction (commonly called a heart attack), or congestive heart failure, where the heart is unable to efficiently pump the blood throughout the body.

Angina pectoris is the result of temporary deprivation of oxygen and often occurs after stress or exertion. Heart attacks occur because a portion of the heart is permanently deprived of blood and the cells become damaged. Congestive heart failure involves a cascade reaction of the body to inefficient heart action that results in accumulation of fluids in the outer reaches of the body. In each of these cases, the trigger cause of the condition was the blockage of the arteries that supply the heart.

A second set of heart diseases involves an abnormality in the electrical system of the heart. Called arrhythmias, these diseases occur when the heart no longer beats in the standard pattern. Altered beat function can greatly reduce the efficiency of the heart and can result in fainting (due to lack of blood to the brain), palpitations (an unpleasant awareness of the beating of the heart), shortness of breath, and chest pain. Some common types of arrhythmia include brachycardia (slow heart beat), atrial fibrillation, and ventricular fibrillation.

Brachycardia is commonly treated using pacemakers, an inplanted device that keeps the heart's rhythm steady. Fibrillations are very fast, inefficient beats of the atrium or ventricles. Fibrillation can be treated with medication or an implanted cardiac defibrillator (ICD) that delivers a shock to the heart to restart normal beating. Arrhythmias can be caused by coronary heart disease, high blood pressure, or a previous heart attack, emphasizing the interrelation of the various heart diseases.

A third kind of heart disease involves damage to one of the four valves of the heart. The frequency of damage to these structures is related to the work that they do—with the structures undergoing the greatest amount of pressure having the highest frequency of disease. Thus, valve problems occurs most frequently with the mitral valve, then the aortic, tricupsid, and pulmonic. Mitral valve prolapse is the most common condition, where excess valve tissue prevents it from closing properly. Surgery may not be necessary, however, until leaking of the valve, known as valve regurgitation, accompanies the prolapse. Regurgitation is a symptom of stenosis, a condition where the valve has become too stiff to function properly.

Untreated rheumatic fever, a bacterial infection, is the most prevalent cause of valve problems. The use of antibiotics to treat strep throat has greatly reduced the incidence of this disease in the United States. Congential defects are the second most common cause of heart valve conditions.

Congenital heart defects, in the valves and other structures, occur when the heart or its vessels do not develop normally before birth. The most common congenital heart defect is a combination of four problems called the teralogy of Fallot. With this problem the ventricular septum is incomplete, there is an obstruction to blood flow beneath the pulmonary artery, the aorta is shifted rightward, and the right ventricular wall is thickened.

A final kind of heart disease is cardiomyopathy. This disorder occurs when the muscle of the heart degenerates. There are multiple causes of cardiomyopathy and it is the number one reason people undergo heart transplants. Categorized by the type of muscle damage, there are three general types of cardiomyopathy: dilated, hypertrophic, and restrictive. Dilated cardiomyopathy refers to the enlargement of the heart that is a response to the overall myocardial weakness. Many problems can cause dilated cardiomyopathy including viral infections, excessive alcohol intake, and myocarditis (inflammation of the heart).

Hypertrophic cardiomyopathy is an abnormal over-growth of the heart muscle. An inherited disease, the overgrown muscle blocks the movement of blood both into and out of the heart. Restrictive cardiomyopathy is due to a stiffening of the heart muscle that prevents it from fully relaxing during diastole. This problem is a symptom of other diseases such as hemochromatosis (a defect in iron use by the body) or amyloidosis (overproduction of antibodies by the bone marrow that cannot be broken down).

Michelle L. Johnson, M.S., J.D.