Which action occurs during ventricular systole?

During ventricular systole, both ventricles of the heart contract. This phase of the cardiac cycle follows atrial systole (when the atria contract) and is critical for pumping blood out of the heart. When the ventricles contract, they generate pressure that forces blood into the aorta from the left ventricle and into the pulmonary artery from the right ventricle. This contraction is essential for maintaining blood circulation throughout the body and ensuring that vital organs receive the oxygenated blood they need.

The action of the ventricles contracting is distinct from the other options provided, which describe different phases of the cardiac cycle or heart function. For instance, both atria contracting occurs during atrial systole, not during ventricular systole. Similarly, the filling of the ventricles happens during diastole, when the heart relaxes and allows blood to flow into the ventricles from the atria. Finally, the heart relaxing is also a characteristic of diastole, as it marks the phase following ventricular systole, allowing the heart chambers to fill with blood again before the next cycle starts.