Cardiovascular System – Heart Anatomy, Function, and Circulatory Pathways Practice Test

During ventricular contraction, the pressure inside the ventricles rises. When it becomes higher than the atrial pressure, the atrioventricular valves snap shut to prevent backflow into the atria. As pressure continues to climb and exceeds the pressure in the aorta and pulmonary trunk, the semilunar valves open and blood is ejected from the ventricles into the systemic and pulmonary circulations. This sequence—AV valves closing and semilunar valves opening—defines the ejection phase of systole. (There is a brief isovolumetric contraction right before the SL valves open, when both sets of valves are closed.)