The tricuspid and bicuspid valves of the heart are also known as atrioventricular valves because they are located between the atria and the venticles of the heart. They prevent blood from flowing back into the atria when the ventricles contract to move the blood either into the pulmonary trunk or the aorta to send the blood through the pulmonary or systemic circulation, respectively. You might also see that the bicuspid valve is called the mitral valve.
Deoxygenated blood returning from the tissues of the body (systemic circulation) returns to the right atrium of the heart via the superior and inferior vena cava. On the left side of the heart, the blood is returning to the atrium from the pulmonary circulation where the blood is reoxygenated at the lungs. The atria (on the right and left sides) then contract and move the blood from atria through the tricuspid (right) and bicuspid (left) valves and into the ventricles. The blood from the right atrium moves into the right ventricle and the blood from the left atrium moves into the left ventricle. The ventricles then contract together, increasing the pressure within the ventricles. The blood moves to an area of lower pressure. This could be back into the atria or into the great vessels (pulmonary trunk and aorta). However, when the ventricles contract and the blood is sent back up toward the atria, the tricuspid and bicuspid valves are pushed shut as the blood pushes against them and the blood is sent instead through semilunar valves into the pulmonary trunk (right side) on its way to the lungs or through the aorta (left side) on its way to the tissues of the body.