Which valves prevent blood from returning to the ventricles after contraction?

The semilunar valves are responsible for preventing blood from returning to the ventricles after they have contracted. These valves are located at the exit points of the ventricles—specifically, the aortic valve at the left ventricle and the pulmonary valve at the right ventricle. When the ventricles contract during systole, blood is pushed out into the aorta and pulmonary artery, and the semilunar valves open to allow this ejection. Once contraction is complete and pressure in the ventricles drops, these valves close, preventing any backflow of blood into the ventricles during the diastolic phase.

The atrioventricular valves, in contrast, are located between the atria and ventricles and serve to prevent backflow into the atria during ventricular contraction. Chordae tendineae are fibrous cords that connect these valves to the ventricular walls, but they do not function as valves themselves. The annulus fibrosus is a fibrous ring that provides support and structure to the heart's valves, but it does not actively prevent blood flow.