into the coronary sinus); Oblique vein of left atrium. All veins that empty into the coronary sinus except for the oblique vein of the left atrium have valves at their junction with the coronary sinus. The anterior cardiac veins do not drain into the coronary sinus but drain directly into the right atrium. Some small veins known as Thebesian veins drain directly into any of the four chambers of the heart. Microanatomy The wall of the coronary sinus is partly muscular. Function The coronary sinus is responsible for venous return of about 55% of the cardiac blood supply. Clinical significance Electrodes can be inserted into and through the coronary sinus to study the electrophysiology of the heart. This includes for a coronary sinus electrogram. The coronary sinus connects directly with the right atrium. It will dilate as a result of any condition that causes elevated right atrial pressure, such as pulmonary hypertension. Dilated coronary sinus is also seen in some congenital cardiovascular conditions, such as persistent left superior vena cava, and total anomalous pulmonary venous return. Additional images See also Coronary arteries Percutaneous coronary intervention Cardiac skeleton References External links – "Posterior view of the heart." Veins of the torso