At the start of a cardiac cycle, all the four chambers of heart are in a relaxed state, i.e., they are in joint diastole. As the tricuspid and bicuspid valves are open, blood from the pulmonary veins and vena cava flows into the left and the right ventricle respectively through the left and right atria. The semilunar valves are closed at this stage. The SAN now generates an action potential, which stimulates both the atria to undergo a simultaneous contraction the atrial systole. This increases the flow of blood into the ventricles by about 30 per cent. The action potential is conducted to the ventricular side by the AVN and AV bundle from where the bundle of Hiss transmits it through the entire ventricular musculature. This causes the ventricular muscles to contract (ventricular systole), the atria undergoes relaxation (diastole), coinciding with the ventricular systole. Ventricle systole pushes blood into the pulmonary artery (right side) and the aorta (left side). The ventricles now relax (ventricular diastole). As the ventricular pressure declines further, the tricuspid and bicuspid valves are pushed open by the pressure in the atria exerted by the blood, which was being emptied into them by the veins. The blood now once again moves freely to the ventricles. The ventricles and atria are now again in a relaxed (joint diastole) state, as earlier.

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