Heart

The heart is a muscular organ located in the chest, positioned between the two lungs and tilted mostly to the left. It functions as a pump: through continuous contraction and relaxation, it propels blood through the vessels, enabling it to reach all the body's tissues.

The human heart has four chambers: two atria (upper chambers that receive blood) and two ventricles (lower chambers that pump blood out). Between the chambers are valves (gates) that ensure blood flows in one direction only and does not flow backward.

Circulation of Blood

Oxygen-poor blood returning from the body enters the right atrium, then the right ventricle, which pumps it to the lungs. The lungs add oxygen and remove carbon dioxide. Oxygenated blood returns to the left side, where the left ventricle—the heart's most muscular chamber—pumps it into the aorta, the largest vessel. This divides circulation into pulmonary circulation and systemic circulation.

Rhythm and Nerve Control

The heart possesses its own system for generating electrical impulses. In the right atrium, a signal originates and spreads throughout the muscle in an orderly pattern. Consequently, the heart can beat even when disconnected from the brain's control, though the nervous and hormonal systems regulate its rate, as when a person runs or is frightened.

Blood Supply to the Heart

The heart muscle receives blood from the coronary vessels on its surface. If one of these vessels becomes blocked, the region it supplies loses oxygen, which is the basis of acute heart attack (infarction). Heart and vessel diseases rank among the world's leading causes of death.