Semi - Lunar valves

So called because of the half-moon shape of the cusps. These valves prevent blood in the pulmonary artery (pulmonic valve) and aorta (aortic valve) from flowing back into the ventricles during ventricular relaxation ( diastole ).

Since these valves will be open during ventricular contraction ( systole ) they do not require a system of papillary muscles and chordaetendinae to prevent valve eversion.