The valve between the right atrium and the right ventricle is called the tricuspid valve. This valve opens to allow blood to be pumped from the right atrium in to the right ventricle. Once the blood has passed through the valve closes so the blood cannot pass back. The name tricuspid comes from the fact that it has three flaps.
The valve between the left atrium and left ventricle is called the bicuspid valve. It also allows the flow of blood in one direction. After the oxygenated blood flows from the atrium to the ventricle the valve closes to prevent the blood from flowing back. This valve has only 2 flaps which give it the name bicuspid.