Final answer:
The aortic and pulmonary semilunar valves each have three leaflets. These valves are essential for preventing the backflow of blood from the arteries to the ventricles and lack chordae tendineae and papillary muscles.
Step-by-step explanation:
The semilunar valves in the human heart, specifically the aortic and pulmonary valves, each have three leaflets. These leaflets are pocket-like folds of endocardium reinforced with additional connective tissue. Unlike the atrioventricular valves, such as the tricuspid and the bicuspid (mitral) valves, the semilunar valves do not have chordae tendineae or papillary muscles. Instead, their structure allows them to prevent backflow of blood from the arteries into the ventricles effectively. When the ventricles relax, a pressure change forces the blood back toward the ventricles, filling the cusps of the semilunar valves and sealing the openings to maintain proper unidirectional blood flow.