The superior vena cava and inferior vena cava drain systemic venous blood into the posterior wall of the right atrium. The internal wall of the right atrium is composed of a smooth posterior portion (into which the vena cavae and coronary sinus drain) and a ridgelike, muscular anterior portion.he opening of the coronary sinus is located between the right AV orifice and the inferior vena cava orifice. The coronary sinus drains all venous blood from the heart except the blood carried from the anterior cardiac veins.