Final answer:
The contraction of papillary muscles during ventricular systole creates tension in the chordae tendineae, preventing the atrioventricular valves from inverting into the atria, thus protecting against regurgitation of blood. They do not function in closing the semilunar valves.
Step-by-step explanation:
Contractions of the papillary muscles serve a crucial role in the functioning of the heart. When the myocardium of the ventricle contracts and the pressure inside rises, the papillary muscles also contract. This action causes tension in the chordae tendineae, which are fibrous cords that connect the papillary muscles to the atrioventricular valves (tricuspid and mitral valves). Therefore, the primary function of the papillary muscles is to prevent the atrioventricular valves from reversing into the atria when the ventricles contract. Without the correct tension, blood could flow backward into the atria, a condition known as regurgitation. Additionally, the contraction of the ventricles triggers the closure of the atrioventricular valves, further preventing backflow.
It is important to note that while the papillary muscles and chordae tendineae are integral to the function of the atrioventricular valves, they are not associated with the semilunar valves (aortic and pulmonary). Instead, the semilunar valves have pocket-like folds of endocardium reinforced with connective tissue, which seal the openings when the ventricles relax, preventing blood from flowing back into the ventricles.