The semilunar valves open during the ejection phase due to the pressure on the left ventricle being higher than the pressure in the aorta and pulmonary artery.
Explanation:
During the ejection phase occurring in the ventricular systole, the pressure in the left ventricle increases and overpasses the pressure in the aorta and pulmonary artery. At this point the semilunar valves open, allowing the blood to flow from the ventricle into the arteries.
