The contraction phase of the cardiac cycle is called:

• A. Systole
• B. Diastole
• C. Isovolumetric contraction
• D. Relaxation

Answer: (A)

During the cardiac cycle, the phase where the heart muscle actively shortens to push blood out is called systole. This is when the ventricles contract, raising pressure enough to open the semilunar valves and eject blood into the aorta and pulmonary artery. A brief part within this phase is isovolumetric contraction, where all valves are closed and pressure rises without a change in volume. The relaxation phase, in contrast, is diastole, when the heart chambers fill with blood. So the contraction phase is systole.