What occurs during the systole phase of the heartbeat?

During the systole phase of the heartbeat, the heart muscle contracts, which is essential for pumping blood throughout the body. This phase is characterized by the contraction of the ventricles, which forces blood into the aorta and pulmonary artery. The heart's contraction generates the pressure needed to circulate blood to the organs and tissues, ensuring they receive the oxygen and nutrients required for proper functioning.

In contrast to this, the other choices refer to different phases or states of the cardiac cycle. For example, relaxation of the heart muscle occurs during diastole, when the heart chambers fill with blood. Atrioventricular valves opening is also part of the diastolic phase when the heart is preparing for the next contraction, allowing blood to flow into the ventricles. Thus, the focus of systole is indeed on contraction and the active phase of the heartbeat, highlighting why the selected answer correctly identifies this critical function.