The mammalian heart is composed primarily of cardiac (or smooth) muscle cells, but includes blood vessels, nerves, and valves. Structurally and functionally, the heart is an efficient, continuously running pump. Arteries, including the aorta (the main artery), carry oxygenated blood away from the heart, and veins return oxygen-depleted blood to the heart.

View a second image of human heart tissue.

From the initiation of development through the moment of death, the heart pumps, requiring tremendous strength, reliability, and resiliency. The average cardiac muscle contracts and relaxes about 80 times per minute. Contractions push the blood through the chambers and into the blood vessels. The innervating neurons regulate the pace of the heart by controlling the muscle contractions. During rest periods such as sleep, the heart pumps more slowly, while during exercise or periods of agitation, the heart pumps more quickly, sending additional oxygenated blood to the body's muscles.

Enclosed in the pericardium, a membranous sac, and protected by the ribcage in front and back, and the diaphragm below, the human heart is characterized by four chambers (two ventricles and two atria) separated by a muscular wall (or septum). Valves connect each atrium to the ventricle below it. Despite the heavy load the heart must bear, the average adult heart is only about the size of a clenched fist and weighs about 285 to 343 grams (10 to 12 ounces) in adult men and 229 to 285 grams (8 to 10 ounces) in adult women.