The human heart is large, hollow muscle that pumps the blood to every part of the body. The heart has four chambers, or 'rooms' , and valves that control the flow of blood. A simple model that you can make shows how the heart pump works. Cut a small hole in a hollow rubber ball. Squeeze the ball press your hands around it, to force the air out of it. Then hold the flattened ball in a pan of water.-------------
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