Heart pumps blood throughout the body supplying oxygen and nutrients so it can be compared to a human pump. pump takes in a substance, usually water, in one end and pushes it out a different end for the purpose of moving the water out of one area and into another.The heart does the same thing. Incoming blood from the body enters the heart in the right and left atria. The atria contract, forcing the blood into the right and left ventricles. That forces the blood towards either the lungs (right ventricle) or out to the body (left ventricle). As with a mechanical pump, blood is moved from the veins (incoming blood) and out to either the lungs to pick up oxygen and get rid of carbon dioxide, or out to the body to send the oxygenated blood out to nourish the cells of the body.