Final answer:
The coronary sinus is a vein that lies in the coronary sulcus, responsible for draining the right atrium and ventricle, returning blood to the heart's right atrium during diastole.
Step-by-step explanation:
The coronary sinus is a large, thin-walled vein located on the posterior surface of the heart within the atrioventricular sulcus. It is responsible for draining the right atrium and right ventricle by collecting blood from the coronary veins, which run parallel to the coronary arteries of the heart. These veins are integral to the cardiac circulation, ensuring deoxygenated blood is returned to the right atrium during diastole, the period when the heart muscle is relaxed.
The coronary sinus receives blood from various coronary veins including the great cardiac vein, the middle cardiac vein, the small cardiac vein, and the posterior cardiac vein. The anterior cardiac veins also help in draining the heart but, unlike others, they bypass the coronary sinus and drain directly into the right atrium. This system ensures that the ejection fraction, which is the portion of blood ejected by the heart with each contraction, maintains efficient circulation within the myocardium.
The coronary sinus lies in the coronary sulcus, drains the myocardium of the heart, and empties deoxygenated blood directly into the right atrium. It is complemented by the anterior cardiac veins which also drain the right ventricle directly into the right atrium without passing through the coronary sinus.