Final answer:
During diastole, the semilunar valves close to prevent backflow into the heart.
Step-by-step explanation:
During diastole, the semilunar valves close to prevent backflow of blood into the heart. The semilunar valves are located at the base of the aorta and the pulmonary trunk. When the ventricular muscle relaxes and the pressure within the ventricles drops below the pressure in both the pulmonary trunk and aorta, the semilunar valves snap shut, preventing blood from flowing back into the heart.