What is the function of AV (atrioventricular valves)?

Atrioventricular valves are two in number. Mitral valve is between the left atrium (upper chamber) and left ventricle (lower chamber). Tricuspid valve is between the right atrium (upper chamber) and right ventricle (lower chamber). Mitral valve closes when the left ventricle contracts, to prevent backflow of blood into the left atrium. Tricuspid valve closes when the right ventricle contracts. Hence the blood from the ventricles are able to go out of the heart into the blood vessels during ventricular contraction. Mitral and tricuspid valves open when the ventricles relax, permitting blood to enter the ventricles from the atria. This blood is pumped out when the ventricles contracts next time.