Openings of the Right Atrium
The right atrium has six openings (inlets and one outlet).
1. Opening of the Superior Vena Cava
- Opens at the upper end of the right atrium (upper part of the sinus venarum)
- No valve
- Returns blood from the upper half of the body
2. Opening of the Inferior Vena Cava
- Opens at the lower end of the right atrium, close to the interatrial septum
- Guarded by a rudimentary, non-functioning semilunar valve — valve of the inferior vena cava (Eustachian valve)
- Returns blood from the lower half of the body
Embryological role: During fetal life, the Eustachian valve directs oxygenated IVC blood (from the placenta) through the foramen ovale into the left atrium, bypassing the pulmonary circulation. After birth it becomes functionless.
Intervenous tubercle of Lower: A small projection on the posterior wall just below the SVC opening. During fetal life, it directs SVC blood towards the right ventricle. Scarcely visible in the adult.
3. Opening of the Coronary Sinus
- Opens between the opening of the IVC and the right AV orifice
- Guarded by a rudimentary, non-functioning valve — valve of the coronary sinus (Thebesian valve)
- The coronary sinus drains most of the venous blood from the heart wall
4. Right Atrioventricular Orifice (Tricuspid Orifice)
- The largest opening in the right atrium
- Lies anterior to the IVC opening
- Communicates the right atrium with the right ventricle
- Guarded by the tricuspid valve (right atrioventricular valve — 3 cusps: anterior, posterior, septal)
- This is the outlet of the right atrium
5. Openings of Venae Cordis Minimae (Thebesian Veins)
- Numerous small foramina scattered in the walls of all four chambers
- Open primarily into the right atrium
- Drain small amounts of blood directly from the myocardium
6. Openings of Anterior Cardiac Veins
- Small veins that open directly into the rough part (atrium proper) of the right atrium — not into the sinus venarum
- Do not drain via the coronary sinus
Summary Table
| Opening | Location | Valve | Notes |
|---|---|---|---|
| SVC | Upper end of sinus venarum | None | Upper body drainage |
| IVC | Lower end, near interatrial septum | Eustachian valve (rudimentary) | Lower body; fetal role — guides blood to foramen ovale |
| Coronary sinus | Between IVC and tricuspid orifice | Thebesian valve (rudimentary) | Main cardiac venous drainage |
| Right AV (tricuspid) orifice | Anterior to IVC | Tricuspid valve (functional) | Outlet — largest opening |
| Venae cordis minimae | Scattered in walls | None | Minor myocardial drainage |
| Anterior cardiac veins | Rough part (atrium proper) | None | Bypass coronary sinus |