Final answer:
The string-like structures that anchor AV valves to papillary muscles are called chordae tendineae, which prevent valve prolapse during ventricular contraction. Semilunar valves, such as the aortic and pulmonary valves, do not have chordae tendineae but function efficiently through pressure changes in the ventricles.
Step-by-step explanation:
The string-like structures that anchor the atrioventricular (AV) valves to the papillary muscles are called chordae tendineae. These are strong strands of connective tissue that extend from the papillary muscles to the valves within the heart, attaching to the AV valves' flaps. The chordae tendineae are vital in preventing the prolapse of these valves during ventricular contraction, ensuring that blood flows properly from the atria to the ventricles without regurgitating back into the atria.
In contrast, the aortic and pulmonary semilunar valves lack these structures because they are not subjected to the same pressures and mechanisms as the AV valves. Semilunar valves rely on pocket-like folds of the endocardium reinforced with connective tissue that operate efficiently based solely on pressure changes as the ventricles relax. When this happens, the blood pushes against the cusps of the semilunar valves, sealing the openings to prevent backflow.