Final answer:
The semilunar valves, including the pulmonary and aortic valves, close during early ventricular diastole to prevent blood from flowing back into the ventricles from the aorta and pulmonary trunk, marking the isovolumic ventricular relaxation phase.
Step-by-step explanation:
During early ventricular diastole, the ventricular muscles relax and the pressure inside the ventricles begins to drop. As the pressure in the ventricles falls below that in the aorta and pulmonary trunk, blood attempts to flow back toward the ventricles, which would lead to backflow. To prevent this, the semilunar valves close. These are comprised of the pulmonary and aortic valves. This moment in the cardiac cycle, where no blood volume changes occur within the ventricles due to closed atrioventricular valves and the closed semilunar valves, is referred to as the isovolumic ventricular relaxation phase, also known as isovolumetric relaxation phase.