Your heart is a hollow muscle about the size of your fist that sits inside your chest.  The heart has four chambers and its job is to circulate blood throughout the body. Two of the chambers receive blood that is returning from the body. The other two chambers work together to pump that blood out. Your heart can change how fast it pumps. When you're sleeping your heart pumps slowly. If you're running your heart will pump very quickly. The heart is truly amazing!