What is the opening between the right atrium and the right ventricle called?

The atrioventricular septum separates the atria and the ventricles. The atria open into the ventricles through the valves.

The valves are muscular flap-like structures that act as doors between the chambers of the heart. They prevent the backward flow of blood between the chambers. The blood from the right atrium flows into the right ventricle through an opening known as the tricuspid valve.