During which phase of the cardiac cycle do the atria contract to fill the ventricles?

The correct answer is atrial systole, which is the phase of the cardiac cycle when the atria contract. During this phase, the contraction of the atria pushes blood into the ventricles, completing the filling process of the ventricles just before they contract. Atrial systole is crucial for ensuring that the ventricles receive adequate blood volume to pump out effectively during the subsequent ventricular systole.

In this context, atrial diastole refers to the phase when the atria are relaxed and filling with blood from the veins, but it is not the time when they contract. Ventricular systole is the phase where the ventricles contract to pump blood out of the heart, while ventricular diastole is when the ventricles relax and fill with blood. Thus, atrial systole is the only phase that specifically describes the contraction of the atria to fill the ventricles with blood.