During which phase of the cardiac cycle are the blood-filled ventricles emptied?

During the cardiac cycle, the phase in which the blood-filled ventricles are emptied is known as systole. This phase involves the contraction of the heart muscle, specifically the ventricles, which leads to the expulsion of blood into the pulmonary artery and aorta.

During systole, electrical impulses from the heart (initiated by the sinoatrial node) trigger the ventricles to contract forcefully, decreasing their volume and increasing intracavitary pressure, allowing blood to be pumped out. This is a crucial part of circulation, ensuring that oxygenated blood reaches the body's tissues and organs while deoxygenated blood is directed to the lungs for oxygenation.

In contrast, the other phases such as diastole focus on the filling of the ventricles with blood, isovolumetric relaxation is the period after ventricular contraction before the heart fills again, and atrial systole refers to the contraction of the atria to push additional blood into the ventricles. Only during systole does the active expulsion of blood from the ventricles occur, making it the correct answer.