As de-oxygenated blood enters the heart in the right atrium, it pumps the blood to push it to the right ventricle and pumps again making the blood move to the lungs where gas exchange occurs, and then the oxygenated blood enter the heart again in the left atrium and pumps to push the blood to the left ventricle and pumps again so that the blood will leave the heart to carry oxygen to the other parts of the body.