The coronary sinus is a short vein, varying in size from about 15 to 65 millimeters, located along the heart's posterior surface between the left ventricle and left atrium. The circumference of the vein is larger than average and is big enough to allow blood to be deposited by most veins that enter the heart including the great cardiac vein, oblique vein and other arterial veins, the left marginal vein, and the posterior veins of the interventricular and left ventricle. The coronary sinus collects the majority of the cardiac venous blood. It receives the blood from the myocardium and facilitates the movement of the blood into the right atrium. The coronary sinus often serves as a marker for surgeons who are performing cardiac surgery. It also plays an important role in many surgical procedures. Balloon catheters can safely be placed here to introduce contrasting agents, cardioplegia buffers, and other therapeutics. For patients suffering from coronary artery disease, it is an effective place to deliver cardioplegia to afford myocardial protection.