The coronary sinus is a large vein which is formed by several smaller veins that converge into one larger blood vessel, opening into the right atrium of the heart. Collecting blood from the myocardium, the coronary sinus is the drainage channel of the deoxygenated blood of heart. It runs across in the groove situated on the posterior surface of the heart, between the left atrium and left ventricle. The coronary sinus collects blood from the small, middle, great and oblique cardiac veins. It also receives blood from the left marginal vein and the left posterior ventricular vein.
The coronary sinus opens into the atrium, between the inferior vena cava and the atrial-ventricular opening. It returns the blood from the substance of the heart, and is protected by a semicircular fold of the lining membrane of the atrium, the coronary valve. The sinus, before entering the atrium, is considerably dilated - nearly to the size of the end of the little finger. Its wall is partly muscular, and at its junction with the great coronary vein is somewhat constricted and furnished with a valve consisting of two unequal segments.