Final answer:
When the ventricles contract and the pressure in each ventricle exceeds the pressure in each atrium, the AV valves close to prevent blood backflow.
Step-by-step explanation:
When the ventricles contract and the pressure in each ventricle exceeds the pressure in each atrium, the atrioventricular (AV) valves close. This prevents blood from being forced back into the atria during ventricular contraction.
During ventricular contraction, the myocardium of the ventricles contracts, causing the papillary muscles to also contract. This generates tension on the chordae tendineae, which are attached to the cusps of the AV valves, helping to hold them in place and preventing them from being blown back into the atria.
Therefore, when the ventricles contract and their pressure exceeds that in the atria, the AV valves close to prevent blood backflow.