Inside the heart the atrioventricular valves (AV) have chordae tendineae to prevent the backflow of blood into the atrium when the ventricle contracts.
Since the AV and semilunar valves are effectively the same thing but in different locations, it would make sense that the semilunar valves also need chords to prevent blood backflow. But they do not. Why doesn't blood flow back into the ventricle after the semilunar valve closes?