Which phase of the cardiac cycle is characterized by the contraction of the ventricles?

The phase of the cardiac cycle characterized by the contraction of the ventricles is known as systole. During systole, the ventricles contract to pump blood out of the heart. This process is critical for maintaining circulation throughout the body. When the ventricles contract, they generate enough pressure to push the blood through the aorta to the rest of the body and through the pulmonary artery to the lungs.

In contrast, diastole is the phase when the heart muscle relaxes and the chambers of the heart fill with blood. The relaxation phase allows the heart to prepare for the next contraction, ensuring there is sufficient blood volume to be pumped out during the next systole. The filling phase refers to the earlier part of diastole, where the ventricles fill with blood from the atria as the heart prepares for the next contraction. Relaxation itself is part of the overall diastolic phase, emphasizing the heart's need to rest between contractions.

Understanding these phases is crucial for grasping how the heart functions effectively in pumping blood throughout the body and maintaining homeostasis. Systole represents a significant phase in the cardiac cycle focused on blood expulsion, thereby distinguishing it as the correct answer.