With both valves closed no blood can leave or enter. This is the period of isovolumetric ventricular contraction . During this phase blood pressure with the ventricle rises but the volume of blood within the ventricle remains the same. Ventricles continue to contract and when the pressure in the left ventricle exceeds the pressure in aorta, the Aortic valve opens resulting in ventricular ejection . Blood can now flow into the aorta and the volume of blood in the left ventricle drops.
Blood volume continues to drop until it reaches the end-systolic volume.
The T wave also occurs during this phase. Ventricular relaxation begins, thus initiating diastole. Very early in this phase ventricular pressure falls below the pressure in the aorta causing the Aortic valve to close . This phase is called isovolumetric ventricular relaxation since the left AV valve is also closed at this time. Ventricular pressure falls below atrial pressure and the left AV valve opens allowing blood to enter the left ventricle again.