When does the AV valve closes during the cardiac cycle?

Not sure how much detail ur looking for here... but basically this is what it is. The tricuspid valve is located between the right atrium (RA) and right ventricle (RV). When the ventricles (right and left) contract, they send blood forward into the arteries (RV to pulmonary artery to send blood to lungs for oxygenation, LV to the aorta to distribute oxygenated blood throughout the body). The purpose of valves is to prevent blood from backflowing. Since you asked about the tricuspid valve, it prevents blood going back up from the RV to the RA. Just in case you don't know the basic heart circulation... deoxygenated blood from the body comes into the RA from the superior and inferior vena cavae, and from the RA goes into the RV. Between these two is the tricuspid valve, and when the RA contracts, blood gets pushed into the RV. When the RV contracts to push blood into the pulmonary artery, the tricuspid valve snaps shut in order to prevent blood from getting pushed up into the RA again. Similarly, there are 3 other valves that function for the same reason (aortic, mitral, and pulmonic). Failure of the valves to close properly causes many heart problems!