What is the term for the phase when semilunar valves open?

The phase when the semilunar valves open is referred to as ventricular ejection. During this phase, the ventricles contract following depolarization, generating enough pressure to overcome the pressure in the aorta and pulmonary artery. This contraction forces blood out of the ventricles, into the systemic and pulmonary circulation, respectively.

In the context of the cardiac cycle, once the pressure in the ventricles exceeds that in the aorta and pulmonary artery, the semilunar valves (the aortic and pulmonary valves) open, allowing blood to be ejected. This is a critical component of the cardiac cycle as it represents the phase when the heart actively pumps blood out, which is a fundamental function of the heart's role in circulation.

The other phases do not involve the semilunar valves opening. Atrial systole occurs when the atria contract to fill the ventricles. Isovolumetric relaxation happens when the ventricles relax and all valves are closed, creating a moment where no blood enters or leaves the ventricles, and ventricular filling is the phase where blood flows passively from the atria to the ventricles before contraction. Hence, the correct term to describe the phase when semilunar valves open is indeed ventricular ejection.