Opening of coronary sinus

The opening of coronary sinus is located into the caudal part of the right atrium, close to the interatrial septum and just dorsal to the opening of the caudal vena cava. The opening is a small, round to oval ostium. It is sometimes guarded by a thin, variably developed valve (valvula sinus coronarii, Thebesian valve in humans), which may be rudimentary or absent. This opening allows venous blood from the myocardium to drain directly into the right atrium.