The coronary sinus is one of the blood vessels which drains deoxygenated blood into the right atrium of the heart. Once the blood enters the right atrium, it can be pumped through the heart and lungs to acquire oxygen so that it can be returned to the circulatory system to supply oxygen to the cells. The circulatory system relies on this cycle, in which blood is continually moved through the body to remove wastes and deliver needed nutrients and oxygen. In the case of the coronary sinus, the vein drains from the coronary veins which run over the heart muscle, known more formally as the myocardium.
At the site where the sinus joins the right atrium, a valve known as the Thebesian valve controls bloodflow into the heart and prevents blood from backing up the coronary sinus. The valve of Thebesius is also sometimes known as the valve of the sinus, and takes the form of a small fold of tissue which allows blood to go one way, but not the other. The heart relies on a series of such one way valves to keep pressure constant and prevent backflow within the circulatory system.
At the point where it reaches the Thebesian valve, the coronary sinus is fairly large as a result of all of the coronary veins which have come together to drain into it. Depending on the person and the circumstances, the vein may be as large as the middle finger. Some patients may have larger or smaller veins on and around the heart, and can have heart muscles of varying sizes as well. Sometimes these variations can be dangerous, while in other instances they are not a cause for concern.
Some people are born with birth defects which involve the coronary sinus. These birth defects may require surgical correction to ensure that the heart will be able to function. Most commonly, the coronary sinus is involved in an atrial septal defect, a medical issue which can lead to complications for the patient if it is not addressed.
The coronary arteries and corresponding sinus can also become occluded. When this occurs, the reason for the blockage needs to be determined with examination and screening so that a doctor can address it. Testing often includes angiography, in which tracers are injected and followed to collect information about the patient's vascular problems. Failure to correct an occlusion can result in damage to the heart.