Interior of the Right Atrium
The interior of the right atrium is divided into three parts:
- Smooth posterior part — sinus venarum
- Rough anterior part — atrium proper (including the auricle)
- Interatrial septum (medial wall)
Crista Terminalis
The crista terminalis is a vertical muscular ridge on the internal surface of the right atrium that separates the smooth posterior part from the rough anterior part.
- Corresponds externally to the sulcus terminalis
- The SA node lies in the upper part of the sulcus terminalis / upper end of crista terminalis
- Musculi pectinati arise from the crista terminalis and fan outwards anteriorly
Smooth Posterior Part — Sinus Venarum
| Feature | Detail |
|---|---|
| Development | Right horn of the sinus venosus |
| Wall | Smooth — no muscular ridges |
| Receives | Most venous tributaries: SVC, IVC (with Eustachian valve), coronary sinus (with Thebesian valve), venae cordis minimae |
| Anterior cardiac veins | Open into the rough part (atrium proper) — not into the sinus venarum |
Rough Anterior Part — Atrium Proper and Auricle
| Feature | Detail |
|---|---|
| Development | Primitive atrium of the embryonic heart tube |
| Wall | Rough — lined by series of transverse muscular ridges — musculi pectinati |
| Musculi pectinati | Arise from the crista terminalis; run forwards and downwards towards the AV orifice, resembling teeth of a comb |
| In the auricle | Musculi pectinati interconnect to form a reticular sponge-like network |
Comparison: Smooth vs Rough Parts
| Feature | Smooth Part (Sinus Venarum) | Rough Part (Atrium Proper) |
|---|---|---|
| Development | Right horn of sinus venosus | Primitive atrium |
| Wall texture | Smooth | Ridged (musculi pectinati) |
| Venous drainage into | SVC, IVC, coronary sinus, venae cordis minimae | Anterior cardiac veins |
| Separated from rough part by | Crista terminalis | Crista terminalis |
Clinical Correlation
The sponge-like interior of the right auricle (reticular musculi pectinati) impedes free flow of blood, predisposing to thrombus formation — particularly during atrial fibrillation. Dislodged thrombi enter the pulmonary circulation → pulmonary embolism.