Heart

B

Generation of the Heartbeat

Unlike most muscles, which rely on nerve impulses to cause them to contract, heart muscle can contract of its own accord. Certain heart muscle cells have the ability to contract spontaneously, and these cells generate electrical signals that spread to the rest of the heart and cause it to contract with a regular, steady beat.

The heartbeat begins with a small group of specialized muscle cells located in the upper right-hand corner of the right atrium. This area is known as the sinoatrial (SA) node. Cells in the SA node generate their electrical signals more frequently than cells elsewhere in the heart, so the electrical signals generated by the SA node synchronize the electrical signals traveling to the rest of the heart. For this reason, the SA node is also known as the heart’s pacemaker.

Impulses generated by the SA node spread rapidly throughout the atria, so that all the muscle cells of the atria contract virtually in unison. Electrical impulses cannot be conducted through the partition between the atria and ventricles, which is primarily made of fibrous connective tissue rather than muscle cells. The impulses from the SA node are carried across this connective tissue partition by a small bridge of muscle called the atrioventricular conduction system. The first part of this system is a group of cells at the lower margin of the right atrium, known as the atrioventricular (AV) node. Cells in the AV node conduct impulses relatively slowly, introducing a delay of about two-tenths of a second before an impulse reaches the ventricles. This delay allows time for the blood in the atria to empty into the ventricles before the ventricles begin contracting.

After making its way through the AV node, an impulse passes along a group of muscle fibers called the bundle of His, which span the connective tissue wall separating the atria from the ventricles. Once on the other side of that wall, the impulse spreads rapidly among the muscle cells that make up the ventricles. The impulse travels to all parts of the ventricles with the help of a network of fast-conducting fibers called Purkinje fibers. These fibers are necessary because the ventricular walls are so thick and massive. If the impulse had to spread directly from one muscle cell to another, different parts of the ventricles would not contract together, and the heart would not pump blood efficiently. Although this complicated circuit has many steps, an electrical impulse spreads from the SA node throughout the heart in less than one second.

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The journey of an electrical impulse around the heart can be traced by a machine called an electrocardiograph (see Electrocardiography). This instrument consists of a recording device attached to electrodes that are placed at various points on a person’s skin. The recording device measures different phases of the heartbeat and traces these patterns as peaks and valleys in a graphic image known as an electrocardiogram (ECG, sometimes known as EKG). Changes or abnormalities in the heartbeat or in the heart’s rate of contraction register on the ECG, helping doctors diagnose heart problems or identify damage from a heart attack.

C

Control of the Heart Rate

In an adult, resting heart rate is normally about 70 beats per minute. However, the heart can beat up to three times faster—at more than 200 beats per minute—when a person is exercising vigorously. Younger people have faster resting heart rates than adults do. The normal heart rate is about 120 beats per minute in infants and about 100 beats per minute in young children. Many athletes, by contrast, often have relatively slow resting heart rates because physical training makes the heart stronger and enables it to pump the same amount of blood with fewer beats. An athlete’s resting heart rate may be only 40 to 60 beats per minute.

Although the SA node generates the heartbeat, impulses from nerves cause the heart to speed up or slow down almost instantaneously (see Nervous System). The nerves that affect heart rate are part of the autonomic nervous system, which directs activities of the body that are not under conscious control. The autonomic nervous system is made up of two types of nerves, sympathetic and parasympathetic fibers. These fibers come from the spinal cord or brain and deliver impulses to the SA node and other parts of the heart.

Sympathetic nerve fibers increase the heart rate. These fibers are activated in times of stress, and they play a role in the fight or flight response that prepares humans and other animals to respond to danger. In addition to fear or physical danger, exercising or experiencing a strong emotion can also activate sympathetic fibers and cause an increase in heart rate. In contrast, parasympathetic nerve fibers slow the heart rate. In the absence of nerve impulses the SA node would fire about 100 times each minute—parasympathetic fibers are responsible for slowing the heart to the normal rate of about 70 beats per minute.

Chemicals known as hormones carried in the bloodstream also influence the heart rate. Hormones generally take effect more slowly than nerve impulses. They work by attaching to receptors, proteins on the surface of heart muscle cells, to change the way the muscle cells contract. Epinephrine (also called adrenaline) is a hormone made by the adrenal glands, which are located on top of the kidneys. Released during times of stress, epinephrine increases the heart rate much as sympathetic nerve fibers do. Thyroid hormone, which regulates the body’s overall metabolism, also increases the heart rate. Other chemicals—especially calcium, potassium, and sodium—can affect heart rate and rhythm.

D

Cardiac Output

To determine overall heart function, doctors measure cardiac output, the amount of blood pumped by each ventricle in one minute. Cardiac output is equal to the heart rate multiplied by the stroke volume, the amount of blood pumped by a ventricle with each beat. Stroke volume, in turn, depends on several factors: the rate at which blood returns to the heart through the veins; how vigorously the heart contracts; and the pressure of blood in the arteries, which affects how hard the heart must work to propel blood into them. Normal cardiac output in an adult is about 3 liters per minute per square meter of body surface.

An increase in either heart rate or stroke volume—or both—will increase cardiac output. During exercise, sympathetic nerve fibers increase heart rate. At the same time, stroke volume increases, primarily because venous blood returns to the heart more quickly and the heart contracts more vigorously. Many of the factors that increase heart rate also increase stroke volume. For example, impulses from sympathetic nerve fibers cause the heart to contract more vigorously as well as increasing the heart rate. The simultaneous increase in heart rate and stroke volume enables a larger and more efficient increase in cardiac output than if, say, heart rate alone increased during exercise. In a healthy adult during vigorous exercise, cardiac output can increase six-fold, to 18 liters per minute per square meter of body surface.

IV

Diseases of the Heart

In the United States and many other industrialized countries, heart disease is the leading cause of death. By far the most common type of heart disease in the United States is coronary heart disease, in which the arteries that nourish the heart become narrowed and unable to supply enough blood and oxygen to the heart muscle. However, many other problems can also affect the heart, including congenital defects (physical abnormalities that are present at birth), malfunction of the heart valves, and abnormal heart rhythms. Any type of heart disease may eventually result in heart failure, in which a weakened heart is unable to pump sufficient blood to the body.

A

Coronary Heart Disease

Coronary heart disease is caused by atherosclerosis, the buildup of fatty material called plaque on the inside of the coronary arteries (see Arteriosclerosis). Over the course of many years, this plaque narrows the arteries so that less blood can flow through them and less oxygen reaches the heart muscle.

The most common symptom of coronary heart disease is angina pectoris, a squeezing chest pain that may radiate to the neck, jaw, back, and left arm. Angina pectoris is a signal that blood flow to the heart muscle falls short when extra work is required from the heart muscle. An attack of angina is typically triggered by exercise or other physical exertion, or by strong emotions. Coronary heart disease can also lead to a heart attack, which usually develops when a blood clot forms at the site of a plaque and severely reduces or completely stops the flow of blood to a part of the heart. In a heart attack, also known as myocardial infarction, part of the heart muscle dies because it is deprived of oxygen. This oxygen deprivation also causes the crushing chest pain characteristic of a heart attack. Other symptoms of a heart attack include nausea, vomiting, and profuse sweating. About one-third of heart attacks are fatal, but patients who seek immediate medical attention when symptoms of a heart attack develop have a good chance of surviving.

One of the primary risk factors for coronary heart disease is the presence of a high level of a fatty substance called cholesterol in the bloodstream. High blood cholesterol is typically the result of a diet that is high in cholesterol and saturated fat, although some genetic disorders also cause the problem. Other risk factors include smoking, high blood pressure, diabetes mellitus, obesity, and a sedentary lifestyle. Coronary heart disease was once thought to affect primarily men, but this is not the case. The disease affects an equal number of men and women, although women tend to develop the disease later in life than men do.

Coronary heart disease cannot be cured, but it can often be controlled with a combination of lifestyle changes and medications. Patients with coronary heart disease are encouraged to quit smoking, exercise regularly, and eat a low-fat diet. Doctors may prescribe a drug such as lovastatin, simvastatin, or pravastatin to help lower blood cholesterol. A wide variety of medications can help relieve angina, including nitroglycerin, beta blockers, and calcium channel blockers. Doctors may recommend that some patients take a daily dose of aspirin, which helps prevent heart attacks by interfering with platelets, tiny blood cells that play a critical role in blood clotting.

In some patients, lifestyle changes and medication may not be sufficient to control angina. These patients may undergo coronary artery bypass surgery or percutaneous transluminal coronary angioplasty (PTCA) to help relieve their symptoms. In bypass surgery, a length of blood vessel is removed from elsewhere in the patient’s body—usually a vein from the leg or an artery from the wrist. The surgeon sews one end to the aorta and the other end to the coronary artery, creating a conduit for blood to flow that bypasses the narrowed segment. Surgeons today commonly use an artery from the inside of the chest wall because bypasses made from this artery are very durable. In PTCA, commonly referred to as balloon angioplasty, a deflated balloon is threaded through the patient’s coronary arteries to the site of a blockage. The balloon is then inflated, crushing the plaque and restoring the normal flow of blood through the artery. See also Coronary Heart Disease.

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