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Joint diastole is a condition when all the four chambers of the heart are at rest or in relaxation. It is also called complete cardiac diastole or early diastole. It takes 0.4 seconds in each cardiac cycle. It is followed by ventricular systole. Atria are already in diastole at this phase, so all the chambers of heart enter the diastolic phase. 

Due to the ventricular diastole, ventricles relax and the pressure in the ventricles decreases than that in the great arteries. To prevent the back flow of blood from great arteries to ventricles, the semilunar valves close rapidly.

During complete cardiac diastole, blood from the superior and inferior vena cava flows into the atria. Gradually, the pressure in the ventricles decreases and finally become lower than atrial pressure. The the auriculoventricular valves open and blood from atria starts entering into relaxing ventricles.

At the end of joint diastole ventricle is almost 70% filled with blood because the atrio ventricular valve is open, rest 30% blood fills when atria contracts.
Correct option is (c).

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