The pressure in the ventricle gradually increases because of the accumulation of blood in the ventricle. The pressure in the atria begins to lower as the atria relaxes. At this time, the ventricle begins to contract. This further increases the pressure in the ventricle so that the pressure in the ventricle now is greater than that in the atria. This results in the closing of the AV valves (due to the blood trying to move back into the atria). The closing of the AV valves is so forceful that it makes a sound (the 'lub' or first heart sound). Th pressure in the ventricle during this stage is still lower than that in the artery so the semi-lunar valves are still closed. Therefore, there is no movement of blood into or out of the ventricle as both sets of valves are closed thus the term isovolumetric.
So in the isovolumetric contraction stage, the Pvent
> Patria so the AV valves are closed and
the Pvent < Partery so the semi-lunar
valves are closed.
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