Pumping Mechanism of the Heart
The heart muscle has two stages, diastole and systole. Diastole is the state of relaxation and systole is the state of contraction.
Blood from the body enters the right atrium through the inferior and superior vena cava. Blood from the lungs enter the left atrium through the pulmonary vein. The heart beat is initiated and maintained by special muscle cells known collectively as the pace maker or sinoatrial node. The pace maker is ideally placed in the muscle of the right atrium. The pacemaker sends a wave of excitation through the muscles of the right atrium and to the muscles of the left atrium. The excitation results in the contraction of both atria (plural for atrium) at the same time resulting in blood rushing across the tricuspid and bicuspid valves into the right and left ventricles.
The wave of excitation travels across the bundle of his in the septum. The wave of excitation then reaches the ventricles which cause contraction at the same time. The tricuspid and bicuspid valves are both closed during contraction of the ventricles causing blood having reached the ventricles, to be forced into the aorta from the left ventricle, and into the pulmonary artery from the right ventricle.
The diagram below shows the diastolic and systolic cycles.