What is the primary physiological activity during mid-diastole?

During mid-diastole, the primary physiological activity in the heart is passive ventricular filling. This phase occurs after the heart's atria have filled with blood and the atrioventricular (AV) valves are open, allowing blood to flow from the atria into the relaxed ventricles.

In this stage of the cardiac cycle, the pressure within the atria is higher than that in the ventricles due to the continuing venous return of blood, which facilitates the flow of blood into the ventricles without the need for atrial contraction. This passive filling is critical for maintaining adequate cardiac output, as it allows the ventricles to fill with blood before the next phase of the cycle, which involves ventricular contraction.

The other choices describe activities that either do not occur or are not the primary focus during mid-diastole. For example, ventricular contraction happens during systole, while atrial contraction occurs at the end of diastole to help complete the filling of the ventricles, and the semilunar valves open during systole to allow blood to exit the ventricles into the arteries.