Final answer:
The pulmonic and aortic valves, or semilunar valves, open when the ventricles contract and close when the ventricles relax to prevent the backflow of blood.
Step-by-step explanation:
The pulmonic and aortic valves in the heart, known collectively as the semilunar valves, open under specific conditions during the cardiac cycle. These valves open when the ventricles of the heart contract in a phase known as ventricular systole.
During this phase, the increased pressure within the ventricles forces the aortic valve and pulmonic valve to open, allowing blood to be ejected into the pulmonary trunk and aorta, respectively. Conversely, when the ventricles relax (ventricular diastole), the pressure in the arteries exceeds that of the relaxed ventricles.
Causing blood to flow back toward the heart and fill the cusps of these valves, which results in the valves closing to prevent backflow into the ventricles.