Cardiac Veins Not Draining into the Coronary Sinus
Most cardiac venous blood drains into the coronary sinus, but two groups of veins bypass the coronary sinus and drain directly into cardiac chambers.
1. Anterior Cardiac Veins
- Number: 3–4 small veins
- Location: Run parallel to each other across the anterior surface of the right ventricle
- Drainage: Open directly into the right atrium through its anterior wall (into the rough part — atrium proper)
- Do not drain via the coronary sinus
2. Venae Cordis Minimae (Thebesian Veins / Smallest Cardiac Veins)
- Extremely small veins present in the walls of all four chambers of the heart
- Open directly into the respective chamber they lie in — without joining any named vein
- Most numerous in the right atrium and right ventricle; fewer on the left side
Why more numerous on the right? The myocardium of the right heart is thinner and less muscular. On the left side, the thicker, higher-pressure myocardium is supplied more completely by the coronary arterial system, leaving fewer direct venous channels needed. The greater number of Thebesian veins on the right side means a small proportion of deoxygenated blood drains directly into the right atrium — a minor physiological right-to-left shunt at the venous level.
Summary: All Cardiac Veins and Their Drainage
| Vein | Drains into |
|---|---|
| Great cardiac vein | Coronary sinus (left end) |
| Middle cardiac vein | Coronary sinus (middle) |
| Small cardiac vein | Coronary sinus (right end) |
| Posterior vein of left ventricle | Coronary sinus |
| Oblique vein of left atrium (Marshall) | Coronary sinus (left end) |
| Left marginal vein | Coronary sinus |
| Right marginal vein | Small cardiac vein OR directly into right atrium |
| Anterior cardiac veins | Directly into right atrium |
| Venae cordis minimae | Directly into respective chamber |
| Coronary sinus itself | Right atrium (between IVC and tricuspid orifice) |