The coronary sinus orifice of the right atrium is the aperture through which most of the venous drainage of the heart is returned to the circulation by the coronary sinus. It is sited in the medial part of the sinus venarum anteromedial to the valve of the inferior vena cava. Anteroinferior to its lies the tricuspid orifice.
The inferior margin of the coronary sinus is lined by a thin sheet of semi-circular endocardium: the valve of the coronary sinus. The valve drops across the coronary sinus orifice whenever the pressure in the right atrium exceeds that in the vein i.e. during atrial contraction.
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