1.8—Cardiac cycle and mammalian heart function

One cardiac cycle alternates atrial systole, ventricular systole and diastole. Pressure changes caused by contraction and relaxation open or close valves, so blood moves in one direction.

During atrial systole, atria push blood into relaxed ventricles. During ventricular systole, atrioventricular valves close and semilunar valves open as ventricular pressure rises. During diastole, ventricles relax and refill when their pressure falls.

If ventricular pressure becomes greater than atrial pressure, the atrioventricular valve closes; if it becomes greater than arterial pressure, the semilunar valve opens. Valve movement follows pressure, not a separate command.

The sequence keeps oxygen-poor and oxygen-rich blood in the correct circuits and gives the heart time to fill between contractions.

Systole means contraction and diastole means relaxation; do not label a phase only by whether blood is entering or leaving, because pressure and valve state determine the flow.