Because our heart has two pump.Each pump has two chambers,the upper and lower chambers.The upper chamber is the atrium that recieves blood coming in from the veins.The lower chamber is the ventricles that forces the blood out into the arteries.There is a valve between each atrium and ventricle to prevent the blood from flowing backwards.The valves are like one-way doors that keep that keep the blood moving in only one direction.Valves control movement of blood into the heart chambers and out to the aorta and the pulmonary artery.
