How do the semi lunar valves work?

The semilunar valves, located at the exit of the heart's ventricles, consist of the aortic and pulmonary valves. They prevent backflow of blood into the ventricles after contraction. When the ventricles contract, the increased pressure forces the valves open, allowing blood to flow into the aorta and pulmonary artery. Once the ventricles relax, the pressure drops, causing the valves to close and prevent blood from returning to the heart.