Final answer:
The AV valves close when pressure in the ventricles increases.
Step-by-step explanation:
The AV valves close when pressure in the ventricles increases. When the ventricles begin to contract, pressure within the ventricles rises and blood flows toward the area of lowest pressure, which is initially in the atria. This backflow causes the cusps of the tricuspid and mitral (bicuspid) valves to close, preventing blood from being forced back into the atria.