Which vessel drains most of the coronary venous blood into the right atrium?

• A. Coronary sinus
• B. Inferior vena cava
• C. Superior vena cava
• D. Azygous vein

Answer: (A)

The heart’s venous blood from the myocardium is collected mainly by a single large channel—the coronary sinus. This vessel sits in the posterior atrioventricular groove and gathers blood from the major cardiac veins (great cardiac vein, middle cardiac vein, small cardiac veins, and others). The coronary sinus then drains its blood into the right atrium, typically at the postero-inferior region, often with a small thebesian valve moderating flow. In contrast, the inferior and superior vena cavae bring systemic venous blood to the right atrium, not coronary venous blood, and the azygos vein drains the posterior thoracic wall into the superior vena cava. A small amount of coronary venous blood can reach the heart chambers directly via thebesian veins, but the vast majority uses the coronary sinus.