Your heart pumps approximately 100,000 times a day. The heart is made up of four chambers that are organized into two separate pumping systems-one on the right and one on the left. The upper chambers are called atria. The lower chambers are the ventricles.Oxygen-poor blood enters the hearts right side-first the atrium, then the ventricle. The right ventricle pumps the blood to the lungs to receive fresh oxygen. From the lungs, the blood flows to the hearts left side-first the atrium and then the ventricle. Finally, the left ventricle walls contract powerfully to push the blood out of the heart and into the body. In this way, a healthy heart ensures that the body has a constant supply of oxygen-rich blood.