The normal healthy heart is a strong, muscular pump a little larger than a fist. It pumps blood continuously through the circulatory system.
The heart has four chambers, two on the right and two on the left:
The right atrium takes in oxygen-depleted blood from the rest of the body and sends it back out to the lungs through the right ventricle where the blood becomes oxygenated.
Oxygen-rich blood travels from the lungs to the left atrium, then on to the left ventricle, which pumps it through the aorta to the rest of the body.
The heart pumps blood to the lungs and to all the body's tissues by a sequence of highly organized contractions of the four chambers. For the heart to function properly, the four chambers must beat in an organized way.
According to the American Heart Association’s “Heart Disease and Stroke Statistics”,