Human heart

HEART The human heart is a specialized, four-chambered muscle that maintains BLOOD flow in the CIRCULATORY SYSTEM. Located in the thorax, it lies left of the body's midline, above and in contact with the diaphragm. It is situated immediately behind the breastbone, or sternum, and between the lungs, with its apex tilted to the body cavity's left side. In most people the apex can be felt during each heart contraction. At rest, the heart pumps …

…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 heart's left half is approximately twice as thick as that of the right half. The thickness of the heart muscle varies from 2 to about 20 mm.