What is cardiac cycle?

Explanation

The cardiac events that occur from the beginning of one heart beat to the beginning of the next, is called cardiac cycle. Cardiac cycle consists of three phases namely atrial systole, ventricular systole and cardiac diastole.
i) Atrial systole : It lasts about 0.1 seconds.
The SAN generate an action potential which stimulate contraction of atria, which helps in the flow of blood into ventricles by about 30%. The remaining blood flows into the ventricles before the atrial systole.
ii) Ventricular systole : It lasts about 0.3 seconds.
Ventricles contract and atria relax during this phase.
Contraction of ventricles raises the pressure in Ventricles due to which AV valves are closed. It causes the first heart sound "Lub".
When pressure in ventricles exceeds the pressure in aortic arches, semilunar valves open. It results the flow of blood from ventricles into aortic arches.
iii) Cardial diastole : It lasts about 0.4 seconds.
The ventricles now relax, atria are also in diastolic condition.
When pressure in ventricles falls below that in aortic arches, semilunar valves are closed.
It causes the second heart sound "dup".
When pressure in ventricles falls below atrial pressure, AV valves open and ventricular filling begins. The total cycle takes about 0.8 seconds. This gives a heart rate of about 75 beats per minute.