The atrial chambers are not simple thin-walled reservoirs. Each has a venous component, a vestibule, an appendage, and a septal surface, but these components differ substantially between the right and left sides. Their muscular architecture is non-uniform and their epicardial relationships determine how activation spreads and how catheters reach common targets. A component-based description provides a clearer framework than describing the atria only by external surfaces. [1-5]

1  Right atrial components

The right atrium receives the superior and inferior caval veins and the coronary sinus within its smooth-walled venous component, the sinus venarum. The terminal crest forms the prominent muscular boundary between this venous component and the trabeculated appendage. Anteriorly and laterally, pectinate muscles arise from the crest and extend through the appendage towards the vestibule of the tricuspid valve. [1-3]

The vestibule is the smooth muscular rim supporting the tricuspid leaflets. Inferiorly, it becomes continuous with the cavotricuspid isthmus. The septal surface contains the oval fossa and its muscular rim, but only the floor of the fossa and its immediate hinge form the true interatrial septum.

2  Right atrial appendage and terminal crest

The right atrial appendage is broad and triangular, with extensive pectinate muscles. Its external crest overlies the precaval segment of the terminal crest. The terminal crest arches anterior to the superior caval orifice, descends along the lateral wall, and ramifies inferiorly near the inferior caval orifice and isthmus. [2]

Thickness and width vary along its course. It is a preferentially aligned muscular structure, but not an insulated conduction tract. The sinus node is related to its superior aspect, and terminal-crest anisotropy can contribute to conduction slowing and atrial tachycardia circuits.

3  Left atrial components

The left atrial venous component is the dominant part of the chamber and receives the pulmonary veins. Its posterior wall and roof are smooth internally but contain myocardium of markedly variable thickness. The vestibule surrounds the mitral orifice. The left atrial appendage is narrow and finger-like compared with the right and contains pectinate muscles confined largely within the appendage. [1,4]

The pulmonary venous orifices lack complete anatomical rings. Myocardial sleeves extend for variable distances onto the veins, with complex circular, oblique, and longitudinal fibre orientation. Intervenous muscular connections may link adjacent veins.

4  Atrial septal surfaces

From the right atrium, the oval fossa appears as a depression surrounded by a muscular rim. Its floor is the flap valve derived from the primary atrial septum. From the left atrium, the free edge of the flap valve may be visible as a crescentic line. The larger surrounding area commonly called septum includes infolded atrial walls containing epicardial fat and is not a true muscular partition. [1]

This distinction is essential for transseptal puncture. The safe target is the thin floor of the oval fossa, while puncture outside it may enter the transverse sinus, aorta, or extracardiac tissue depending on direction.

5  Major atrial muscular bundles

Atrial fibres are organised into overlapping bundles rather than uniform layers. Bachmann bundle crosses the anterior interatrial groove and provides a broad preferential route towards the left atrial roof and appendage. Posteriorly, muscular continuity also exists through the septal and coronary sinus regions. [1,5]

On the left atrial roof and posterior wall, the septopulmonary and septoatrial bundles contribute to changes in fibre orientation. Abrupt transitions, wall-thickness variation, and intersections between bundles influence activation and lesion formation. These structures should be described as preferentially aligned myocardial bundles, not as isolated cables.

6  External relations and clinical relevance

Anteriorly, the right atrium is related to the ascending aorta and right ventricular inlet, while the left atrium lies mainly posteriorly. The transverse sinus separates the atrial roof from the arterial trunks. Posteriorly, the left atrium is related to the oesophagus and descending aorta, with pericardial and fatty tissues interposed to variable degrees.

For electrophysiology, the component anatomy clarifies the location of the sinus node, terminal crest, cavotricuspid isthmus, triangle of Koch, pulmonary veins, appendages, and septal puncture site. Detailed treatment of each target should remain in its dedicated chapter.