The heartbeat is what pumps blood. The heartbeat is a series of contractions of the myocardium. These contractions are stimulated by nerve cells, which are stimulated by blood flow. A cluster of nerve cells called the sinoatrial node, located in the upper portion of the right atrium, initiate contractions of the atria. A cluster of nerve cells, called the atrioventricular node, located in the lower portion of the right atrium, initiate contractions of the ventricles.
Contractions push blood on to the next location.
Therefore, the sinoatrial node pushes blood from the right atrium to the right ventricle, and from the left atrium to the left ventricle. The atrioventricular node pushes blood from the right ventricle to the pulmonary artery, and from the left ventricle to the aorta. The contraction phase of the heartbeat, when blood is being pushed out, is called the systole . The relaxation phase of the heartbeat, when blood is filling the chambers, is called the diastole . Arteries push blood away from the heart.