Coronary sinus

The coronary sinus is a dilation of the cardiac vein system that collects deoxygenated blood from the heart muscle (myocardium). It delivers this blood to the right atrium. 

The coronary sinus opens into the right atrium through the orifice of the coronary sinus, located between the inferior vena cava and the right atrioventricular orifice. It is protected by a semicircular fold of the lining of the auricle called the valve of coronary sinus (or valve of Thebesius). 

Before entering the right atrium, the coronary sinus is considerably dilated - nearly to the size of the end of the little finger. Its wall is partly muscular. At its junction with the great cardiac vein, the coronary sinus is somewhat constricted and has a valve consisting of two unequal segments.