The human heart is a specialized, four chambered muscle that maintains blood flow in the circulatory system. The heart is located in the thorax, it lies left of the body's midline, above and in contact with the diaphragm. It is behind the breastbone, or sternum, and between the lungs, with its apex tilted to the body's cavity left side. In most people the apex can be felt during each heart contraction. When the heart is at rest, the heart pumps about 59cc (2 oz) of blood per heart beat and 5 l (5qt) per minute, compared to 120-220cc (4-7.3oz) per beat and 20-30 l (21-23 qt) per minute during exercise. The adult human heart is about the size of a fist and weighs about 250-350 gm (9oz).

Blood supplies food and oxygen to the cells of the body for their life needs and removes the waste products of their chemical processes. It also helps to maintain a consistent body temperature, circulate hormones, and flight infections. The brain cells are very dependent on a constant supply of oxygen. If the circulation to the brain is stopped, death ensues shortly. Since heart attacks are the number-one cause of death in the United States, the heart gets a great deal of attention.

The heart's wall has three parts, Muscle tissue, or myocardium, is the middle layer. The inner layer, or endocardium, that lines the inside of the heart muscle consists of a thin layer of endothelial tissue overlying a thin layer of vascularized connective tissue. The outside of the heart, the epicardium, is in intimate contact with the pericardium; this serous membrane is a closed sac covering the heart muscles outside wall. Within the sac, a small amount of fluid reduces the friction between the two layers of tissue. In addition to, muscular and connective tissue, the heart muscle contains varying amounts of fatty tissue, especially on the outside. Both anatomically and functionally, the heart is divided in to a left and a right half by the cardiac septum. Each half contains two separate spaces: the atrium, or auricle, and the ventricle. The upper reservoirs, or collecting chambers, are the thin-walled atria, and the lower pumping chambers are the thick-walled ventricles. The total thickness of the ventricular walls is about three times that of the atria; the wall of the hearts left half is approximately twice as thick as that of the right half. The thickness of the heart muscle varies from 2 to about 20mm (0.1 to 0.8in). This thickness is correlated with the maximum pressure that can be attained in each chamber.

The heart muscle pumps the blood by means of rhythmical contractions and dilations. The hearts left and right halves work almost synchronously. When the ventricles contract, the valves between the atria and the ventricles close, as the result of increasing pressure, and the valves to the pulmonary artery and the aorta open. When the ventricles become flaccid during diastole and the pressure decreases, the reverse process takes place: through the valves between the atria and the ventricles, which are now open again, blood is drawn from the atria into the ventricles, and the valves to the pulmonary artery and the aorta close.