Final answer:
The presence of chordae tendineae and papillary muscles is more critical for atrioventricular valves than semilunar valves, preventing backflow of blood into the atria.
Step-by-step explanation:
The presence of chordae tendineae and papillary muscles is more critical for the atrioventricular (tricuspid and mitral) valves than the semilunar (aortic and pulmonary) valves.
Chordae tendineae are fibrous cords that connect the valves to the papillary muscles. When the ventricles contract, the papillary muscles also contract, generating tension on the chordae tendineae. This prevents the flaps of the atrioventricular valves from being forced into the atria and decreasing the risk of backflow.
On the other hand, the semilunar valves lack chordae tendineae and papillary muscles. Instead, they consist of pocket-like folds of endocardium reinforced with connective tissue. The pressure changes in the ventricles during contraction and relaxation are sufficient to close and open the semilunar valves without the need for the extra support provided by chordae tendineae and papillary muscles.