Final answer:
The pressure in the left ventricle must be greater than the pressure in the aorta for the aortic semilunar valve to open and blood to be ejected. This is part of the ventricular ejection phase where the stroke volume is ejected, and the left ventricle generates enough pressure to overcome the higher aortic pressure.
Step-by-step explanation:
The ejection of blood from the left ventricle through the semilunar valve will begin once pressure in the ventricle is greater than pressure in the aorta. This occurs during the ventricular ejection phase of the cardiac cycle. When the ventricular muscles contract, they raise the pressure within the ventricle higher than the pressure in the aorta, causing the aortic semilunar valve to open and allowing blood to be ejected into the aorta. The left ventricle, which is responsible for pumping blood to the systemic circuit, needs to generate a significant amount of pressure to overcome the high pressure in the aorta.
The stroke volume, which is the volume of blood ejected by the ventricle during each contraction, typically ranges from 70-80 mL, with an end systolic volume (ESV) of 50-60 mL remaining in the ventricle after the contraction. It is important to note that both the left and right ventricles pump the same amount of blood despite the left ventricle facing a higher pressure in the aorta compared to the pressure faced by the right ventricle in the pulmonary trunk.