What is the heart?
All the cells in the human body are dependent on the blood in the cardiovascular system (the heart and blood vessels) for the transport of gases, nutrients, hormones, and other factors. Likewise, the tissues must have a way to dispose of waste products so that they do not build to harmful levels. All these substances are dissolved in the blood, but something must provide the force to transport the blood to all parts of the body at all times—the heart. This organ must beat continuously from early in development to death. It beats without conscious control and can vary how quickly it moves blood throughout the body depending on the needs and activities of the tissues.
In humans, an individual’s heart is about the size of his or her fist and is enclosed in the center of the chest cavity between the lungs. The heart contains specialized muscle cells known as cardiac muscle. These cardiac cells make up most of the thickness of the walls of the heart; they are responsible for moving blood out of the heart and are also involved in maintaining the rhythm of the heartbeat. This heavily muscled layer is referred to as the myocardium. The inner lining of the heart is called the endocardium; it is continuous with the lining of all the blood vessels in the body. The outermost layer of the heart is the epicardium, which covers the myocardium. The heart moves as it beats and is contained within a fluid-filled bag called the pericardial sac. The rhythmically beating heart has the potential to rub against adjacent structures (such as the lungs), harming itself and those structures. Therefore, it is important that the heart be encased in the pericardial sac, with its lubricating fluid.
The human heart has four separate chambers. These internal cavities can be identified by their location and function. The upper pair of smaller chambers are known as atria, and the lower larger chambers are called ventricles. Because the atria and ventricles have a muscular wall which separates them into right and left halves, one can refer to the individual chambers as the right atrium and left atrium, and the right ventricle and left ventricle. The wall that separates the right and left halves of the heart is called the septum. The septum prevents any mixing of blood from the right and left sides of the heart. The atria and ventricles on the same side, however, must allow blood to pass between them in a single direction. This action is accomplished by one-way valves between the atria and ventricles. The valve that allows blood to pass from the right atrium to the right ventricle is called the tricuspid valve because it is made of three flaps. On the left side of the heart is the bicuspid valve (with two flaps), which is also known as the mitral valve. The bicuspid valve allows blood from the left atrium to flow only into the left ventricle. This rather complex anatomy is necessary because the heart must pump blood in one direction and into two separate systems.
The anatomy of the heart often makes more sense if one understands its function or physiology. As an example, one may consider an active cell in the body, perhaps a muscle cell that moves the foot. This cell utilizes oxygen to help metabolize food for energy. During this process, carbon dioxide is produced as a waste product, and high levels of carbon dioxide can be harmful to cells. Therefore, one of the jobs of the cardiovascular system is to deliver oxygen and take away carbon dioxide. Once the carbon dioxide is picked up by the blood, it travels back to the heart via veins and enters the right atrium. From the right atrium, the blood passes the tricuspid valve and enters the right ventricle. The right ventricle then sends the blood past a one-way semilunar valve called the pulmonic valve into blood vessels that transport it to the lungs. At the lungs, the blood loses carbon dioxide and picks up oxygen. This oxygenated blood must now be delivered to the tissues. First, the blood returns to the heart and enters the left atrium. From the left atrium, blood is pushed past the bicuspid mitral valve into the left ventricle. The blood is then pumped from the powerful left ventricle through another semilunar valve (the aortic) into the blood vessels that will carry the blood to all the tissues of the body, including the heart itself. The blood vessels that feed the heart directly are known as coronary vessels.
The orderly pattern by which blood flows through the heart, lungs, and body requires the chambers of the heart to work in a coordinated fashion. The atria contract together to help send blood into the ventricles. The ventricles then contract together so that blood flows through the lungs from the right ventricle and through the tissues of the body from the left ventricle. The tricuspid and bicuspid valves prevent a backflow of blood into the atria when the ventricles contract, and the semilunar valves prevent blood from returning to the ventricles after they have contracted.
Something must coordinate the contraction of the heart so that the atria contract together before the ventricles do so. Highly specialized cells of the myocardium have the ability to conduct electrical impulses rapidly and to discharge spontaneously at a certain rate. These properties allow the heart to be stimulated in a synchronous way and for it to generate its own rate and rhythm. One region of the right atrium is known as the sinoatrial (S-A) node; it functions as the heart’s pacemaker. The S-A node has the ability to generate spontaneously an electrical signal with a relatively rapid rhythm. Therefore, it serves to “pace” the heart rate. When the S-A node sends its electrical impulse throughout the atria, the atria contract. There is a slight delay before the impulse reaches the ventricles, which allows the atria to contract fully before the ventricles. The atrioventricular (A-V) node will then pick up the electrical signal and send it through both ventricles via specialized conductive heart muscle fibers known as Purkinje’s fibers. Purkinje’s fibers transmit the electrical signal ensuring that all the ventricular muscle cells contract at nearly the same time. The ventricles contract in such a way that the bottom tip of the heart (apex) contracts slightly before the region of the ventricles next to the atria (base). Additionally, the ventricles contract in a somewhat twisting motion that causes the heart to “wring out” the blood.
This rather complex system allows the heart to contract at its own rate and in a highly synchronous fashion. Nevertheless, one’s heart rate varies depending on one’s physical activity or emotional state. For example, during exercise or when an individual is under stress, the heart rate goes up. When one is relaxed, the heart does not beat as rapidly. Therefore, the body must have a way to regulate the rate at which the S-A node signals the heart to contract.
The autonomic nervous system, which functions without one’s conscious control, regulates the heart rate. It is divided into two systems: parasympathetic and sympathetic. The parasympathetic nervous system is active during periods of rest and has the ability to slow the heart. During periods of physical or emotional stress, the sympathetic nervous system stimulates the heart to contract more forcefully and at a more rapid rate. The parasympathetic and sympathetic systems communicate with the heart via chemical messengers known as neurotransmitters. The parasympathetic nervous system uses the neurotransmitter acetylcholine to slow the heart, while norepinephrine and epinephrine are the chemicals used by the sympathetic nervous system to increase the heart rate.
Even though the heart seems to be adaptable to a variety of situations throughout one’s life, it can malfunction. In fact, diseases of the heart and blood vessels are the number-one killer in the United States. One common disease that affects the heart directly is coronary artery disease, which can lead to life-threatening heart attacks. Although medical researchers are still investigating the causes of coronary artery disease, most of the evidence points to hypertension (high blood pressure) and atherosclerosis (a buildup of fatty plaque in the walls of arteries).
Hypertension is usually defined as a blood pressure greater than 140/90 millimeters of mercury (mmHg) at rest. A typical blood pressure for a young, healthy adult is 120/80 mmHg. The top number measures the force of blood against an artery wall during the contraction of the heart; this is referred to as the systolic pressure. The bottom number, the diastolic pressure, is a measurement of force when the heart is relaxed. If either systolic or diastolic pressure exceeds 140/90 mmHg, the patient is considered hypertensive. The cause of hypertension has not been determined, but it is known that with hypertension the heart must work harder to push the blood through the arteries, including the coronary arteries. Physicians treat hypertension by prescribing drugs that block the effect of the sympathetic nervous system on the heart, such as metoprolol (Lopressor). They may also prescribe drugs such as prazosin (Minipress) that prevent the arteries from becoming too narrow.
Hypertension is also seen in patients who have atherosclerosis. This buildup of fatty materials such as cholesterol under the lining of the artery causes the plaque to protrude, narrowing the diameter of the vessel. This can lead to blood clot formation on artery walls that are irregular. This clot, also known as a thrombus, may dislodge and travel in the bloodstream. Eventually, it may block a small artery, thereby preventing the flow of blood to the tissue. If this happens in a coronary artery, a myocardial infarction (heart attack) will result.
A heart attack occurs when a portion of the heart dies because of a lack of oxygen or a buildup of waste products. Heart muscle has no way of repairing itself, and the resulting damage is permanent. If the patient is transported to the hospital immediately, the emergency room physician may give drugs to prevent further blood clot formation (aspirin and heparin) and to help dissolve the already formed clot (reteplase and tenecteplase-tissue plasminogen activator, or TNK-TPA). If the coronary artery is only partially blocked, the patient may suffer from angina pectoris, a chest pain that radiates down the left arm. These patients usually take drugs such as nitroglycerin, which help dilate (widen) blood vessels, reestablishing adequate flow to the heart.
Another devastating disease of the heart is congestive heart failure, a condition in which the heart fails to pump enough blood to meet the demands of the body’s tissues. The heart becomes enlarged because of the resulting excessive increase in blood volume. There are several causes of heart failure, most of which stem from the fact that the heart loses its ability to pump efficiently. For example, a patient who has had a heart attack may have lost significant function as a result of heart damage. Even without a heart attack, some individuals may have malfunctioning heart valves or other problems that cause an inefficient ejection of blood and thus heart failure.
The cardiovascular system attempts to compensate for heart failure in several ways. The sympathetic nervous system increases the heart rate, and the kidneys retain more fluid to increase blood volume. These compensatory mechanisms help to reestablish adequate blood flow for a while. Because of the increase in blood volume, however, more blood enters the chambers of the heart and causes them to stretch. At some point, the ventricles can no longer force out the increased amount of blood entering them, and they enlarge. This increase in the size of the heart chamber further enlarges the heart and strains the heart muscle. The heart will continue to weaken, unable to keep up with the body’s demands. Compensatory mechanisms attempt to meet the body’s need for continuous blood flow but in doing so further overload the heart. This vicious circle may lead to complete heart failure and death.
Congestive heart failure may involve only one side of the heart, perhaps because of a heart attack that affected that side. If the heart failure occurs on the left side, the right ventricle is pumping blood to the lungs in an efficient manner but the left ventricle cannot pump all the blood returning from the lungs. Therefore, blood backs up and pools in the lung tissues. Similarly, if the right ventricle begins to fail and the left ventricle is normal, blood begins to pool throughout the body since the right side of the heart cannot keep up in its pumping.
Physicians are able to slow the progression of congestive heart failure by prescribing drugs such as digoxin that increase the force of heart muscle contraction and thereby the amount of blood ejected with each beat. Therapeutic agents such as captopril (Capoten) help to reduce the fluid retention in the kidneys.
Heart failure is related to the inability of the heart to contract well due to coronary artery disease or other conditions. In addition, the specialized heart muscle cells that provide the heart’s rhythm and conduct the electrical signals necessary for a coordinated heartbeat may be affected by disease. In the resting adult, the heart normally beats about seventy to eighty times per minute. Several conditions exist whereby the heart loses control of its normal rate and rhythm, a serious condition.
For example, if the heart begins to beat too rapidly, the ventricles do not have enough time to fill and the movement of blood to the heart muscle and the rest of the body is impaired. The atria or ventricles may contract at a high rate and lose their coordinated sequence of contraction; this is referred to as atrial or ventricular fibrillation. Atrial fibrillation may be tolerated under some circumstances; ventricular fibrillation is a medical emergency. If immediate action is not taken to reestablish the normal rate and conduction sequence, the patient will die. Emergency measures such as electrical defibrillation may shock the heart into reestablishing its normal rhythm and conduction pathways. It is easy to understand how these abnormal patterns of heart activity occur if one imagines more than one pacemaker attempting to control heart function. The cause of these and other, less severe heart rhythms may be heart damage affecting the conductive pathway, drugs, or even psychological distress.
Heart disease is a major cause of death, but most experts agree that many heart problems are preventable. High blood pressure and high blood levels of fat and cholesterol are associated with an increased incidence of coronary artery disease. Cigarette smoking and excessive weight are also correlated with heart disease. Additionally, exercise seems to be critical in maintaining a healthy heart, as sedentary individuals have a twofold increase in their risk of heart disease when compared to active people.
It is likely that individuals who are at risk can lessen the probability of having heart problems by adopting a more healthful lifestyle, including stopping smoking, reducing excessive weight and mental stress, and engaging in enjoyable physical activities (with their physicians’ permission).
The role of the heart in the functioning of the human body was questioned by the ancient Egyptians, who attributed breathing to the heart. It was the Chinese who first documented that the heart is responsible for the pulse and movement of blood. They also believed that the heart was the seat of happiness. The ancient Greeks had a different idea about the function of the heart, believing that it was the region where thinking originated.
It was not until William Harvey (1578–1657), an English physiologist, published his experiments on the heart and circulation that scientists believed blood was pumped continuously by the heart. He observed that both ventricles of the heart contracted and expanded at the same time. Harvey also noted that when the heart was removed from an animal, it continued to contract and relax; that is, it had an automatic rhythm.
More than one hundred years after Harvey published his work, Stephen Hales made the first blood pressure measurements. He did so by inserting a tube into the neck artery of a horse and watching the blood rise 3 meters above the animal. Then early in the twentieth century Willem Einthoven invented an instrument to measure electrical currents. This instrument was used by Thomas Lewis to measure the electrical activity in the heart, the first electrocardiograph (ECG).
By the mid-twentieth century, heart surgeries were being performed to correct heart defects. These early surgeries had to be done with the heart still beating. In 1953, the heart-lung machine was used to take over the pumping function of the heart during surgery so that the surgeon could stop the heart. In 1967, Christiaan Barnard performed the first heart transplantation in a human. Heart transplants were performed during the next ten years with no long-term survivors, usually because of tissue rejection. In 1982, a completely artificial heart was implanted into a patient. This patient died in the spring of 1983.
Heart transplants have become much more successful, however, mainly because of the use of immunosuppressive drugs that help to prevent rejection of the transplanted heart. In January 2012, the US National Heart, Lung, and Blood Institute reported that heart transplant patients had a one-year survival rate of 88 percent, a five-year survival rate of 75 percent, and a ten-year survival rate of 56 percent. Newer drugs and procedures such as coronary bypass surgery, angioplasty, and atherectomy are becoming more effective in treating heart disease. Nevertheless, perhaps the best approach to maintaining a healthy heart is to practice preventive medicine. Scientists are making comparable strides in finding ways to prevent heart disease as they are in treating already existing conditions.
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