As the ventricle continues to contract, the pressure builds up until the pressure in the ventricle eventually becomes greater than the pressure in the artery. At this point, the semi-lunar valves will swing open due to the rush of blood from the higher pressure ventricle to the lower pressure artery. Blood then moves into the artery under high pressure, which is the purpose of the heart. The ventricular pressure is now very high and much greater than that in the atria so the AV valves are still closed.

So in the ejection stage, the P_vent < P_atria so the AV valves are closed and
the P_vent > P_artery so the semi-lunar valves are open..

Go to Isovolumetric Relaxation