[dr_ravi]

hello thankx for question..Did you mean ventricular contraction and ventricular relaxation.
At first you must understand this :
SAN -> atria (systole) -> AVN -> bundle of His -> bundle branches – ventricles (systole)


Ventricular Diastole
At the end of ejection, ventricular volume is at its minimum and ventricular muscle fibers begin to relax.
1. Ventricular repolarization occurs (T wave), marking the end of contraction and beginning of relaxation.

2. The linear velocity of blood flow in the aorta falls (pressure in the aorta exceeds pressure in the ventricle and the pressure gradient is reversed), then flow is transiently reversed and blood returns toward the ventricle.

3. The aortic valve now closes . Momentum of blood returning to the ventricle from the aorta causes the aortic valve to bulge into the ventricle, then return to its normal position. This oscillation causes the incisura or dicrotic notch in the aortic pressure recording.

4. Ventricular pressure falls very rapidly, but volume does not change because no valves are open. This is the period of isovolumic relaxation, and its end is marked by the opening of the mitral valve .

5. Maximum atrial pressure has been achieved (also maximum atrial volume, causing the "v" wave. Atrial pressure exceeds ventricular pressure, the AV valve opens, and blood rapidly enters the relaxing ventricle (this is the rapid filling phase. Pressure in the atrium falls ("y" descent), but remains above that in the ventricle until a subsequent ventricular contraction occurs. There follows a period of reduced filling , then a period of further reduced filling referred to as diastasis .