At the beginning of systole, when ventricular pressure exceeds atrial pressure, the AV valves close. However, initially the semilunar valves also remain closed since ventricular pressure is still not sufficiently strong enough to force them open (consequently at this point in the cardiac cycle there is no blood flow into or out of the ventricles). This phase is known as . A) isovolumetric relaxation (phase 4)
B) isovolumetric contraction (phase 2)
C) ventricular ejection (phase 3)
D) ventricular filling (phase 1)