 At this stage, which corresponds to the R peak or the QRS complex seen on an ECG, the semilunar valves (aortic and pulmonary valves) are also closed.The net result is that, while contraction causes ventricular pressures to rise sharply, there is no overall change in volume because of the closed valves.The isovolumetric contraction phase lasts about 0.03 s, but this short period of time is enough to build up a sufficiently high pressure that eventually overcomes that of the aorta and the pulmonary trunk upon opening of the semilunar valves.