The regular beating (about 70 times per minute) of the heart is regulated by specialized heart muscle cells which produce electrical impulses and stimulate the heart to contract. These collections of cells, the sinoatrial and atrioventricular nodes, are called pacemakers.
The inside of the heart is divided into two parallel and completely separate pumps lying side-by-side, generally referred to as the left side and the right side of the heart. The left side of the heart receives oxygenated blood from the lungs, and then pumps this oxygenated blood to all the tissues of the body. The right side of the heart receives de-oxygenated blood from the rest of the body, and then pumps it back to the lungs where it loses carbon dioxide and picks up oxygen. Despite their proximity, the left and right sides of the heart do not exchange blood.
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