Most of the veins of the heart drains the oxygen-poor blood into a reservoir which is called coronary sinus. The coronary sinus lies in the left posterior portion of the atrioventricular groove, measuring between 2 and 3 cm in length. Being a continuation of the great cardiac vein, it receives deoxygenated blood from the oblique vein of the left atrium, the posterior vein of the left ventricle, as well as from the middle cardiac and the small cardiac vein. Finally, the coronary sinus empties all this returning blood from the heart tissue into the right atrium, with the blood flowing through the valve of the coronary sinus.
The great cardiac vein begins on the anterior surface of the apex of the heart, having as tributaries the veins of the anterior wall of both ventricles. Running up obliquely to the atrioventricular groove, it fits into it. When it reaches the diaphragmatic part of the atrioventricular groove, it widens to become the coronary sinus. The oblique vein of the left atrium arises on the lateral wall of the left atrium and then it runs down from left to right as a small branch in a fold in the pericardium; descending to the right, it then empties into the coronary sinus.
The anterior cardiac veins are not tributaries of the coronary sinus. Running obliquely upwards across the anterior-lateral surface of the right ventricle of the heart, they drain the oxygen-poor blood directly into the right ventricle.
Below, anterior side of heart, showing its veins and arteries.
Below, the posterior aspect of heart, showing the great cardiac veins, as it turns around the left lateral border of this organ, and the coronary sinus, with the tributary veins.
