Which best describe the isovolumetric contraction phase of the cardiac cycle? which best describe the isovolumetric contraction phase of the cardiac cycle? as ventricular systole starts, the av valves are closed and the semilunar valves are closed. because the ventricles are contracting and both valves are closed, pressure increases rapidly leading to ejection. as ventricular systole starts, the av valves close, which immediately opens the semilunar valves. the pressure from the atrial contractions opens the semilunar valves. as pressure builds up in the ventricles during systole, the av valves open and allow blood to leave the heart.