Cardiac cycle is the event during, which one heartbeat or one cycle of contraction (systole) and relaxation (diastole) of cardiac muscle occurs. A cardiac cycle consists of systole and diastole of atria and systole and diastole of ventricles. The time of cardiac cycle is in reverse ratio of the rate of heartbeat. In man, the heart rate is about 72 times $/ \mathrm{min}$, therefore time of a cardiac cycle is $60 / 72=0.8 \mathrm{sec}$ approx .