The heart beats rhythmically in a sequence called the Cardiac Cycle, a rapid coordination of contraction, systole, and relaxation, diastole. An electrical signal sent from the sinoatrial node, near the right atrial wall causes both atria to simultaneously contract and push blood into the ventricles.

When the pulse reaches the atrioventricular node between the right atrium and right ventricle, it pauses for approximately a 1/10th of a second which allows blood to completely empty from the atria.

The charge then spreads through the Bundle of His, down the intraventricular septum by way of the right and left bundle branches and then through the walls of the ventricles' Purkinje fibers inducing ventricular contraction and pumping blood out of the heart and into the pulmonary artery and aorta.

Meanwhile, the atria which are in diastole, continue to be filled with blood. As the ventricles relax, the blood pours down into the ventricles and the cycle continues.