Final answer:
The semilunar valves, which include the aortic and pulmonary valves, open when the ventricles contract to allow blood ejection into arteries and close to prevent backflow when the ventricles relax.
Step-by-step explanation:
The semilunar valves are part of the heart's valve system and include the aortic valve and the pulmonary valve. These valves allow blood to be ejected from the heart into the arteries and prevent backflow into the ventricles. Specifically, they are forced open when the ventricles contract, ejecting blood into the pulmonary trunk and aorta.
Emerging from the right ventricle is the pulmonary semilunar valve, comprised of three flaps that close when the pressure in the ventricle becomes lower than the pulmonary trunk, preventing backflow. The aortic semilunar valve, located at the base of the aorta, functions similarly by preventing backflow from the aorta into the left ventricle.
Unlike the atrioventricular valves, semilunar valves do not have papillary muscles or chordae tendineae. During times when the ventricles are not contracting, the semilunar valves remain closed to maintain one-way blood flow through the heart and out to the body and lungs.