Cardiac Cycle

The cardiac cycle comprises systole, the contraction phase, and diastole, the relaxation phase. During systole, the heart chambers contract and eject blood. During diastole, the chambers relax and refill with blood. These alternating phases repeat with each heartbeat, regulated by pressure gradients that control blood flow through the heart.

During atrial systole, a cardiac action potential from the sinoatrial node triggers atrial contraction, ejecting blood into the ventricles. This phase lasts approximately 0.1 seconds. After atrial systole completes, the atria enter diastole and begin refilling with blood while the ventricles prepare to contract.

Isovolumetric contraction is the phase when both atrioventricular and semilunar valves remain closed, maintaining constant ventricular blood volume. This occurs after atrial systole when the cardiac potential triggers ventricular contraction. Rising ventricular pressure closes the atrioventricular valves, and the ventricles contract without ejecting blood until pressure exceeds arterial pressure.

Approximately 60 mL of blood, called the end-systolic volume, remains in the ventricles after contraction. This leftover blood is not ejected because ventricular pressure eventually falls below arterial pressure, causing the semilunar valves to close and preventing further blood ejection during that heartbeat.

The semilunar valves open when ventricular pressure exceeds arterial pressure during ventricular systole. This pressure gradient forces the valves open, allowing blood to be ejected into the aorta and pulmonary trunk. Once ventricular pressure falls below arterial pressure during diastole, the valves close to prevent backflow.

Isovolumetric relaxation occurs during early diastole when all four heart valves are closed, maintaining constant ventricular blood volume. This phase begins after ventricular systole ends and repolarization reduces ventricular pressure. Arterial backflow triggers semilunar valve closure, initiating this relaxation phase before the atrioventricular valves reopen.

A complete cardiac cycle lasts approximately 0.8 seconds in an average heart beating around 75 times per minute. Atrial systole accounts for 0.1 seconds, ventricular systole for 0.3 seconds, and total heart relaxation for 0.4 seconds. This timing ensures efficient blood flow and adequate filling between contractions.