Right upper chamber; receives venous blood from body + coronary circulation → right ventricle via tricuspid orifice.

Position

• Behind + right of right ventricle

Receives: SVC, IVC, coronary sinus, anterior cardiac veins, venae cordis minimae

External Features

• Main cavity + right auricle

Right auricle — small muscular appendage; projects left, overlaps ascending aorta root; notched margins; musculi pectinati (sponge-like) inside

Rough interior promotes thrombus in AF

Sulcus terminalis — vertical groove; SVC opening (above) to IVC opening (below); internally = crista terminalis; SA node at upper end near SVC opening

Right AV groove — right coronary sulcus; contains right coronary artery + small cardiac vein

Interior — 3 Parts

1. Smooth posterior: sinus venarum
2. Rough anterior: atrium proper + auricle
3. Interatrial septum

Crista terminalis — vertical ridge; separates smooth/rough parts; origin of musculi pectinati; upper end relates to SA node

Sinus venarum (smooth) — from right horn of sinus venosus; openings: SVC, IVC, coronary sinus, venae cordis minimae

Rough anterior part — from primitive atrium; musculi pectinati from crista terminalis

Openings (6 groups)

Interatrial Septum

• Fossa ovalis, annulus ovalis/limbus, triangle of Koch, torus aorticus

Fossa ovalis — shallow oval depression, lower septum; site of fetal foramen ovale; floor = septum primum; thinnest part of septum

Annulus ovalis (limbus) — raised margin; well marked above/sides, deficient inferiorly; free edge of septum secundum

Patent foramen ovale — closes after birth (LA pressure > RA pressure); if patent: usually functionally closed; may open if RA pressure rises → paradoxical embolism

Triangle of Koch — AV node landmark

AV node at/near apex; landmark for ablation, respected in tricuspid surgery

Torus aorticus — anterosuperior septal elevation from non-coronary aortic sinus bulge

Development

Applied Anatomy

• ASD — usually fossa ovalis region; left-to-right shunt; RA+RV enlargement; fixed split S2
• PFO — usually asymptomatic; paradoxical embolism risk if RA pressure exceeds LA
• Atrial thrombus — rough auricular cavity favours clot in AF
• AV nodal procedures — triangle of Koch locates AV node for ablation/surgery

The right atrium is the right upper chamber of the heart. It receives venous blood from the body and coronary circulation and sends it to the right ventricle through the tricuspid orifice. For exams, the key areas are its external features, internal parts, openings, fossa ovalis, and triangle of Koch.

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Position

The right atrium lies behind and to the right of the right ventricle.

It forms:

It receives:

• Superior vena cava
• Inferior vena cava
• Coronary sinus
• Anterior cardiac veins
• Venae cordis minimae

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External Features

Shape and Parts

The right atrium is roughly quadrilateral and elongated vertically.

It has:

1. Main cavity
2. Right auricle

Right Auricle

The right auricle is a small muscular appendage projecting from the upper part of the right atrium.

Features:

• Projects to the left.
• Overlaps the root of the ascending aorta.
• Has notched margins.
• Interior contains musculi pectinati forming a sponge-like network.

Clinical importance:

• The rough auricular interior may promote thrombus formation in atrial fibrillation.

Sulcus Terminalis

The sulcus terminalis is a vertical groove on the external surface of the right atrium.

Extent:

• From the opening of SVC above
• To the opening of IVC below

Importance:

• Corresponds internally to the crista terminalis.
• The SA node lies at the upper end of the sulcus terminalis near the opening of the SVC.

Right Atrioventricular Groove

This is the right part of the coronary sulcus, separating right atrium from right ventricle.

Contents:

• Right coronary artery
• Small cardiac vein

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Interior of the Right Atrium

The interior is divided into three parts:

1. Smooth posterior part: sinus venarum
2. Rough anterior part: atrium proper and auricle
3. Interatrial septum

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Crista Terminalis

The crista terminalis is a vertical muscular ridge inside the right atrium.

Functions / importance:

• Separates smooth and rough parts internally.
• Corresponds externally to sulcus terminalis.
• Gives origin to musculi pectinati.
• Upper end is related to the SA node.

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Smooth Posterior Part: Sinus Venarum

The anterior cardiac veins open mainly into the rough part, not into the sinus venarum.

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Rough Anterior Part

Musculi pectinati arise from the crista terminalis and pass forwards and downwards like the teeth of a comb.

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Openings of the Right Atrium

The right atrium has six groups of openings.

Superior Vena Cava Opening

• Opens into upper posterior part of right atrium.
• Has no valve.
• Returns blood from upper half of body.

Inferior Vena Cava Opening

• Opens into lower part of right atrium near interatrial septum.
• Guarded by the rudimentary Eustachian valve.
• In fetal life, this valve directs oxygenated IVC blood through the foramen ovale.

Coronary Sinus Opening

• Lies between IVC opening and right AV orifice.
• Guarded by the Thebesian valve.
• Drains most venous blood from the heart wall.

Right Atrioventricular Orifice

• Largest opening of right atrium.
• Opens into right ventricle.
• Guarded by tricuspid valve.

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Interatrial Septum

The interatrial septum forms the medial wall of the right atrium.

Important features:

1. Fossa ovalis
2. Annulus ovalis / limbus
3. Triangle of Koch
4. Torus aorticus

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Fossa Ovalis

The fossa ovalis is a shallow oval depression in the lower part of the interatrial septum.

Importance:

• Represents the site of the fetal foramen ovale.
• Floor is formed mainly by septum primum.
• It is the thinnest part of the interatrial septum.

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Annulus Ovalis

The annulus ovalis, or limbus fossa ovalis, is the raised margin around the fossa ovalis.

Features:

• Well marked above and on the sides.
• Deficient inferiorly.
• Represents free edge of septum secundum.

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Patent Foramen Ovale

The foramen ovale normally closes after birth when left atrial pressure rises above right atrial pressure.

If it remains patent:

• It is usually functionally closed.
• It may open when right atrial pressure rises.
• It may permit paradoxical embolism, causing stroke or systemic embolism.

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Triangle of Koch

The triangle of Koch is an important landmark for the AV node.

The AV node lies at or near the apex of the triangle of Koch.

Clinical importance:

• Landmark in electrophysiological procedures.
• Important in AV nodal ablation.
• Must be respected during tricuspid valve surgery to avoid heart block.

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Torus Aorticus

The torus aorticus is an elevation in the anterosuperior part of the interatrial septum produced by the bulging of the non-coronary aortic sinus.

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Developmental Summary

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Applied Anatomy

Atrial Septal Defect

Defects in the interatrial septum commonly involve the region of the fossa ovalis.

Effects:

• Left-to-right shunt
• Right atrial and right ventricular enlargement
• Fixed splitting of second heart sound

Patent Foramen Ovale

Usually asymptomatic but may allow paradoxical embolism when right atrial pressure exceeds left atrial pressure.

Atrial Thrombus

The rough auricular cavity may favour clot formation in atrial fibrillation.

AV Nodal Procedures

The triangle of Koch is used to locate the AV node during catheter ablation and cardiac surgery.

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