What does systole refer to in heart function?

Systole refers specifically to the phase of the cardiac cycle during which the heart's ventricles contract. During this phase, the muscles of the ventricles constrict to pump blood out of the heart and into the arteries—namely, the aorta for the left ventricle and the pulmonary artery for the right ventricle. This contraction is essential for maintaining effective blood circulation throughout the body, as it allows oxygenated blood to reach various tissues and organs.

In contrast, the other terms provided describe different aspects or phases of heart function. For example, ventricular relaxation refers to diastole, where the heart chambers fill with blood, while atrial relaxation occurs as the heart prepares for the next contraction. Blood volume increase is a result of filling during diastole rather than a direct function of systole itself. Therefore, defining systole as ventricular contraction captures its primary role in the cardiac cycle.