The heart is a hollow muscular organ that receives blood from the veins, circulates it through the lungs, and propels it back into the body through the arteries. The human heart is approximately the size of a closed fist and is located behind the lower part of the sternum (breastbone), extending slightly to the left of the chest's midpoint.
The heart is held in place principally by its connection to the great arteries and veins and by its containment in the pericardium, a double-walled sac with one layer enveloping the heart and the other attached to the breastbone, diaphragm and membranes of the thorax.
Blood carries oxygen to all the cells so they can function and stay healthy. It also washes away carbon dioxide, the waste product produced by the cells as they do their work. The average adult body contains approximately 5.5 quarts of blood, and the heart circulates that blood supply about 1,000 times each day.
The adult heart has two parallel independent systems, each composed of an atrium and a ventricle. From their anatomical placement, these systems are referred to as the right and left heart chambers. Blood coming from the head and body, filled with carbon dioxide and very little oxygen, enters the right auricle through two large veins, the superior and inferior venae cavae. Also, the blood that has supplied the heart itself drains directly into the right atrium. This return of venous blood to the heart takes place during the entire heart cycle of contraction and relaxation, and to the right ventricle only during the relaxation, or diastole, part of the cycle.
Near the end of diastole, contraction of the right auricle fills the right ventricle through the tricuspid valve, then rhythmic contractions of this ventricle during systole expels the blood through the pulmonary arteries into the lungs where the blood is oxygenated. The lungs' capillaries return the blood to the left atrium via the pulmonary veins. The blood passes through the mitral valve into the left ventricle, the largest and strongest pumping chamber of the heart. It delivers the oxygen-rich blood through the aorta to the smaller arteries, and thus back to the cells throughout the body. The blood forced from the ventricles during systole is prevented from returning during diastole by valves at the openings of the aortic and pulmonary arteries.
The heart beats because of an electric current that passes through its cells. The rate of heartbeat is controlled by the autonomic nervous system, accelerated by the sympathetic system and slowed by the parasympathetic system. Nervous electric impulses originate from the sinus node, a bundle of nerve cells located in the right atrium near the superior vena cava. Each time the sinus node sends out an electric signal, the heart cells along its path contract, causing the pumping action of the heart. These impulses stimulate the atria to squeeze first, and then activate the ventricles.
The heart produces two sounds in each cycle of the beat. The first is rather dull and caused by vibration of the atrioventricular valves, and by the contraction of ventricular muscle fibers. The second is a sharper sound caused by the sudden closing of the aortic and pulmonary valves. In cardiac disease these regular sounds may be accompanied or replaced by murmurs caused by turbulent blood rushing through abnormal valves or openings.Heart Failure
Disorders of the heart kill more Americans than any other disease. They can arise from congenital birth defects, bacterial or viral infections, narrowing of the coronary arteries, high blood pressure, alcohol abuse or disturbances of heart rhythm. The major form of heart disease in Western countries is atherosclerosis. Atherosclerosis restricts blood flow to the heart because fatty deposits called plaque have built up in the coronary arteries. The term heart failure does not mean that the heart has totally stopped working. Actually, the heart is still working, but it is just not able to pump enough blood through the body to maintain vital organs and other functions as well as it should.
Heart failure is a condition that impacts people of all ages and walks of life, from children and young adults to the middle-aged and elderly. It is a progressive disease that often results from damage or injury to the heart. The major culprits include coronary artery disease, hypertension, cardiomyopathy, heart attack, birth defects, arrhythmias, and valve disease. One of the hallmarks of this disease is the buildup of fluid in the body and organ tissues, also referred to as congestive heart failure. At this stage, the buildup of fluid will cause the heart to move less blood throughout the body with each heartbeat, causing fluid to back up in the lungs. Heart failure also affects the ability of the kidneys to dispose of sodium and water.
Symptoms of heart failure are sometimes subtle and may be mistaken for other ailments, which is why this condition is usually not detected until it has become more advanced. Because the heart is not circulating enough blood throughout the body, a person with heart failure may experience swelling (or edema) in the legs, ankles and feet, and may notice swelling in other parts of the body as well. Additional symptoms to watch for include shortness of breath, limitation of physical activity, fatigue, and frequent coughing.Treatment
Heart transplantation is the surgical placement of a healthy heart from a human donor (a cadaveric organ) into the body of a person whose own heart is badly diseased. The procedure is also referred to as orthotopic cardiac graft. It is performed when congestive heart failure or heart injury cannot be treated by any other medical or surgical means. Transplantation is reserved for those individuals with a high risk of dying from heart disease within one or two years. Most patients who undergo a transplant have one of two problems:
- Irreversible damage to the heart caused by coronary artery disease, commonly called "hardening of the arteries," and multiple heart attacks.
- Cardiomyopathy, or heart muscle disease. In this condition, the heart cannot contract normally because of damage to the muscle cells. It may have been caused by bacterial or viral infection or hereditary factors.
Occasionally, heart transplants are performed on people with other forms of heart disease. These might include the effects of rheumatic fever or hypertension (high blood pressure), abnormalities in the heart valves that cause damage to the heart muscle, congenital heart defects, those structural abnormalities present at birth, or rare conditions like heart tumors.
The immune system intervenes in the recovery process and remains a topic of concern for patients following transplantation. One type of immunity, called adaptive immunity, occurs when components of the immune system - the lymph nodes, thymus, spleen and bone marrow and the cells they generate - work in concert to develop defenses specifically tailored to particular structures, called antigens, on the surface of a "foreign" invader, such as a new organ. Adaptive immunity responds only after the invader is present, developing a specific response to the particular antigen. It also displays "memory" so it can defend against repeat assaults from the same antigen even years later.
Therefore, the immune system recognizes and attacks anything different from the substances normally present in the body, even those only slightly different, such as a transplanted heart. This response is known as rejection. To minimize the chances of transplant rejection, physicians seek donor organs that share as many biochemical properties as possible with the transplant recipients. Even then, transplant recipients must take medications - immunosuppressants - to suppress their natural immune response and prevent rejection.