The heart is a hollow muscular organ the size of a fist. It lies in the center of the chest, slightly to the left and under the sternum or breastbone.
The heart’s function is to pump blood containing oxygen and nutrients to all parts of the body. The heart is divided into four chambers (two atriums and two ventricles). The right side of the heart receives "used" blood from the body and pumps it to the lungs, where it picks up oxygen. The oxygen-rich blood then returns to the left side of the heart and is pumped through the aorta to all parts of the body.
There are four valves in the heart. These are located between the upper and lower chambers of the heart and in the major blood vessels of the heart. The valves keep blood pumping in one direction.
At rest, a heart beats an average of 60–100 times a minute. Each beat or contraction is caused by an electrical stimulus triggered by specialized cells called the SA node. The electrical impulse from the SA node travels through a series of fibers located throughout the heart chamber and causes a contraction of the heart muscle.