The coronary sinus is a collection of smaller veins that merge together to form the sinus (or large vessel), which is located along the heart's posterior (rear) 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. The coronary sinus collects the majority of the cardiac venous blood. It receives the blood from the myocardium, a thick layer of muscle within the heart, and facilitates the movement of the blood into the right atrium. The coronary sinus often serves as a landmark for surgeons who are performing cardiac surgery. It also plays an important role in many other heart procedures. For example, balloon catheters can safely be placed here to introduce contrasting agents and other therapeutics. For patients suffering from coronary artery disease, the coronary sinus an effective place to deliver cardioplegia to protect the myocardium from damage during surgery. Cardioplegia refers to the intentional, temporary cessation of heart activity (heartbeat) during cardiac surgery.