The organ that pumps the blood for circulating throughout the human body.

The human heart is a pulsating, four-chambered organ that circulates blood through the body. Though the heart appears to be a simple organ, it requires a complex series of nerve stimulations, valve openings, and muscle contractions to adequately achieve its purpose.

The human heart on the average weighs about 10.5 oz (300 g). Cone-shaped and about the size of a closed fist, it 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 miles (96,000 km) of vessels to nourish each of the trillions of cells in the body. Each contraction forces about 2.5 oz...