Cardiac Cycle

The cardiac cycle comprises the contraction, or systole, and relaxation, or diastole, during one heartbeat.

During atrial systole, cardiac action potential from the sinoatrial node triggers atrial contraction, ejecting blood into the ventricles, following which the atria enter diastole.

At the end of ventricular diastole, the ventricles contain about 130 mL of blood, called the end-diastolic volume.

The cardiac potential traveling down the atrioventricular node triggers ventricular systole.

As the ventricles contract, the rising pressure closes the atrioventricular valves. This phase is called isovolumetric contraction, where all valves remain closed, maintaining a constant ventricular blood volume.

Eventually, ventricular pressure exceeds arterial pressure, prompting the semilunar valves to open and eject blood. The leftover blood, about 60 mL, called the end-systolic volume, stays in the ventricles.

During ventricular diastole,  repolarization reduces the pressure, triggering arterial backflow, closing the semilunar valves, and leading to isovolumetric relaxation of the ventricles.

While the ventricles were contracting, the atria were being filled with blood. Eventually, the atrial pressure exceeds the ventricular pressure, restarting the cycle.