The human heart is a remarkable organ. Although it is only about the size of a clenched first, the heart is perhaps the strongest organ in the body, and is certainly one of the hardest workers. The heart is made up of four chambers, the left and right auricles and the left and right ventricles. There is a valve, like a sort of trap door, between the auricle; and the ventricle on each side, which prevents the blood returning to the chamber it has just left. There are also valves between the auricles and the ventricles and the major blood vessels which are attached to the heart. Although the heart has been described as a pump, it is in fact two pumps.
The left side of the heart is the first of these. It receives blood from the lungs which has a plentiful supply of oxygen in it, and it sends this blood through the arteries to the rest of the body. The right side of the heart of receives blood back from the body. Now, it has less oxygen in it and a lot of carbon dioxide waste from the cells which the body must get rid of. When this blood returns through the veins, the heart sends it to the lungs, where it get rid of the carbon dioxide and stocks up with oxygen again as we breathe. The heart pumps the blood by squeezing itself up and relaxing. You have no control over this; the heart pumps blood around your body about 100,000 times a day, and it does so very efficiently.