The coronary sinus () is the largest vein of the heart. It drains over half of the deoxygenated blood from the heart muscle into the right atrium. It begins on the backside of the heart, in between the left atrium, and left ventricle; it begins at the junction of the great cardiac vein, and oblique vein of the left atrium. It receives multiple tributaries. It passes across the backside of the heart along a groove between left atrium and left ventricle, then drains into the right atrium at the orifice of the coronary sinus (which is usually guarded by the valve of coronary sinus). Structure Origin The coronary sinus arises upon the posterior aspect of the heart between the left atrium, and left ventricle. The coronary sinus commences at the union of the great cardiac vein, and the oblique vein of the left atrium. The origin of the coronary sinus is marked by the Vieussens valve of the coronary sinus which is situated at the endpoint of the great cardiac vein. Course The coronary sinus runs transversely in the left atrioventricular groove (coronary sulcus) on the posterior aspect of the heart. The sinus, before entering the right atrium, is considerably dilated - nearly to the size of the end of the little finger. Fate The coronary sinus drains through the posterior wall of right atrium at the orifice of the coronary sinus. This orifice is located at the posteroinferior aspect of the right atrium, just medial and to the left of the orifice of inferior vena cava, and between the inferior vena cava and the right atrioventricular orifice/tricuspid valve. The orifice is usually guarded by the valve of coronary sinus (a thin, crescent-shaped valve located on the anteroinferior part of the opening into the right atrium). Tributaries The coronary sinus receives blood mainly from the small, middle, great, and oblique cardiac veins. It also receives blood from the left marginal vein and the left posterior ventricular vein. Great cardiac vein (run upwards in the anterior interventricular sulcus to the left atrioventricular groove to form the coronary sinus; Middle cardiac vein (ascends posterior interventricular sulcus to drain into coronary sinus); Small cardiac vein (accompanies right coronary artery in the right atrioventricular groove to drain into the right side of the coronary sinus; Posterior vein of left ventricle (acccompanies the left marginal artery, ascends the posterior wall of left ventricle to drain