Heart

Located in the thoracic cavity, the heart is a four-chambered muscular organ that serves as the primary pump or driving force within the circulatory system. The heart contains a special form of muscle, appropriately named cardiac muscle, that has intrinsic contractility (i.e., is able to beat on its own, without nervous system control).

The Chinese were aware more than 2,000 years ago that the heart is a pump that forces blood through a maze of arteries. The Greeks, however, believed the blood did A scanning electron micrograph (SEM) of the aortic valve. It lies between the left ventricle of the heart and the aortic arch and consists of a fibrous ring with three semilunar pockets attached to it. Photograph by Prof. P. Motta/G. Macchiarelli University "La Sapienza," Rome/Science Photo Library, National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission.
not circulate at all, and their ideas dominated medical science until the seventeenth century. The Greek physician Galen published a great deal on the human body and its functions, much of which was incorrect, but his doctrines held sway for hundreds of years. Not until the 1600s did William Harvey, through human and animal experiments, discover that the heart circulates the blood. He published his findings in 1628, thus bringing Western medical science in line with that of the ancient Chinese.

The human heart on the average weighs about 10.5 oz (300 g). It is a four-chambered, cone-shaped organ about the size of a closed fist that lies in the mid-thorax, under the breastbone (sternum). Nestled between the lungs, the heart is covered by a fibrous sac called the pericardium. This important organ is protected within a bony cage formed by the ribs, sternum, and spine.

In its ceaseless work, the heart contracts some 100,000 times a day to drive blood through about 60,000 mi (96,000 km) of vessels to nourish each of the trillions of cells in the body. Each contraction of the ventricles forces about 2.5 oz (0.075 l) of blood into the circulation, which adds up to about 10 pt (4.7 l) of blood every minute. On average, the heart will pump about 2,500 gal (9,475 l) of blood in a day, and that may go up to as much as 5,000 gal (18,950 l) with exertion. In a lifetime the heart will pump about 100 million gal of blood.

The chambers of the human heart are divided into two upper (superior) atrial chambers and two thickerwalled, heavily muscular inferior ventricular chambers. The right and left sides of the heart are divided by a thick septum. The right side of the heart is on the same side of the heart as is the right arm of the patient. The atrial and ventricular chambers on each side of the septum constitute separate collection and pumping systems for the pulmonary (right side) and systemic circulation (left side). The coronary sulcus or grove separates the atria from the ventricles. The left and right side atrial and ventricular chambers each are separated by a series of one way valves that, when properly functioning, allow blood to move in one direction, but prohibit it from regurgitating (flowing back through the valve).

Deoxygenated blood—returned to the heart from the systemic circulatory venous system—enters the right atrium of the heart through the superior and inferior vena cava. Auricles lie on each atrium and are most visible when the atria are drained and deflated. The auricles (so named because they resembled ear flaps) allow for greater atrial expansion. Pectinate muscles on the auricles assist with atrial contraction. Small contractions within the right atrium, and pressure differences caused by evacuation of blood in the lower (inferior) right ventricle, cause this deoxygenated blood to move through the tricuspid valve during diastole (the portion of the heart's contractile cycle between contractions, and a period of lower pressure as compared to systole) into the right ventricle. When the heart contracts, a sweeping wave of pressure forces open the pulmonic semilunar valve that allows blood to rush from the right ventricle into the pulmonary artery where it is travels to the lungs for oxygenation and other gaseous exchanges.

Freshly oxygenated blood returns to the heart from the pulmonary circulation through the pulmonary vein the empties into the left atrium. During diastole, the oxygenated blood moves from the left atrium into the left ventricle through the mitral valve. During systolic contraction, the oxygenated blood is pumped under high pressure through the semilunar aortic valve into the aorta and thus, enters the systemic circulatory system.

As the volume and pressure rise during the filling of the right and left ventricles, the increased pressure snaps shut the flaps of the atrioventricular valves (tricuspid and mitral valves) anchored by fibrous connection to the left and right ventricles. The pressure in the ventricles seals the valves and as the pressure increases during systole, the valves seal becomes further compressed. A prolapse in one of the valves (a pushing through of one of the cusps) leads to blood flow back through the valve. The cusps are held against prolapse by the chordae tendineae, thin cords that attach the cusps to papillary muscles.

The heart and great vessels attached to it are encased within a multi-layered pericardium. The outer layer is fibrous and covers a double membraned inner sac-like structure termed the pericardial cavity that is filled with pericardial fluid. The pericardial fluid acts to reduce friction between the heart, the pericardial membranes, and the thoracic wall as the heart contracts and expands during the cardiac cycle.

The heart muscle is composed of three distinct layers. The outermost layer, the outer epicardium, is separated from the inner endocardium by the middle pericardium. The outer epicardium is continuous and in some places the same as the visceral pericardium. Epicardium protects the heart and is invested with capillaries, nerves, and lymph vessels. The middle myocardium is a think layer of cardiac muscle. The innermost endocardium contains connective tissue and Purkinje fibers. The endocardium is continuous with the lining of the great vessels attached to the heart and it lines all valve and cardiac inner surfaces.

Heart muscle does not directly take up oxygen from the blood it pumps. A specialized set of vessels (e.g., the left and right coronary arteries and their branches) supply oxygenated blood to the heart muscle and constitute the coronary circulation. A heart attack occurs whenever blood flow is occluded (blocked).

The fossa ovalis is a remnant or the embryonic foramen ovale that allows blood to flow between the left and right atria in the developing fetus.