Right Atrium

Introduction

The right atrium (RA) is one of the four cardiac chambers and plays a crucial role in collecting systemic venous blood and channeling it into the right ventricle. It is often underestimated in its importance, yet its structural integrity, conduction system elements, and hemodynamic function are essential for maintaining normal cardiac output and rhythm. In this detailed article, we explore the anatomy, venous inflow, interatrial septum, SA node location, physiology, blood flow dynamics, and clinical relevance of the right atrium, along with imaging and histology insights.

1. Anatomical Overview of the Right Atrium

1.1 Location & Orientation

The right atrium lies in the right anterior portion of the heart and forms the right border of the cardiac silhouette on a chest radiograph. It sits anterior to the esophagus and posterolateral to the ascending aorta.

1.2 Shape & General Description

It is a quadrangular chamber with a smooth posterior part (sinus venarum) and a trabeculated anterior part (atrium proper). The two regions are separated internally by the crista terminalis — a muscular ridge running vertically from the superior vena cava (SVC) orifice to the inferior vena cava (IVC) orifice.

1.3 Walls & Surfaces

Anterior wall: Mostly trabeculated due to pectinate muscles, continuous with the right atrial appendage (auricle).
Posterior wall: Smooth-walled sinus venarum, where venous blood drains.
Lateral wall: Houses the crista terminalis and the openings of small veins.
Medial wall: Interatrial septum, containing the fossa ovalis.
Roof: Receives the opening of the SVC.
Floor: Contains the IVC opening and coronary sinus orifice.

1.4 Borders

Right border: Formed almost entirely by the right atrium.
Left border: Separated from the left atrium by the interatrial septum.
Superior border: Continuous with the SVC.
Inferior border: Continuous with the IVC and tricuspid valve orifice.

2. Venous Inflow System of the Right Atrium

The right atrium serves as the primary collecting chamber for systemic venous return. Four main inflow routes deliver blood into the RA:

2.1 Superior Vena Cava (SVC)

Drains venous blood from the upper body (head, neck, upper limbs, thorax).
Opens into the superior part of the posterior wall.
Has no valve at its orifice.
Clinical relevance: SVC syndrome (obstruction leads to venous congestion and collateral circulation development).

2.2 Inferior Vena Cava (IVC)

Drains blood from the lower body.
Opens into the inferior portion of the posterior wall.
Guarded by the Eustachian valve, a crescent-shaped fold (vestigial in adults).
Directs blood toward the foramen ovale in fetal life (important for right-to-left shunt during fetal circulation).

2.3 Coronary Sinus

Opens between the IVC orifice and the atrioventricular orifice.
Receives almost all venous blood from the myocardium.
Orifice guarded by Thebesian valve.
Important in electrophysiological procedures (catheter placement for pacing or ablation).

2.4 Thebesian Veins (Venae Cordis Minimae)

Numerous small veins that drain directly into the RA cavity.
Though minor contributors, they are part of normal myocardial venous return.

3. Interatrial Septum & Fossa Ovalis

3.1 Interatrial Septum

Thin wall separating right and left atria.
Composed of a central depression (fossa ovalis) and surrounding muscular rim (limbus fossae ovalis).

3.2 Fossa Ovalis

Represents the closed remnant of the foramen ovale of fetal circulation.
Important as a potential site of interatrial communication (Patent Foramen Ovale, PFO).
Clinically significant in paradoxical embolism, cryptogenic stroke, and transseptal puncture procedures.

4. SA Node Anatomy & Location

4.1 Anatomy

The sinoatrial (SA) node is the heart’s natural pacemaker.

Located subepicardially at the junction of the SVC and RA near the superior end of the crista terminalis.
Spindle-shaped, 15 mm long, 3 mm wide.
Supplied by the SA nodal artery (from right coronary artery in ~60%, from circumflex artery in ~40%).

4.2 Histology

Specialized pacemaker cells (P cells) with few myofibrils, rich in glycogen, capable of spontaneous depolarization.

4.3 Clinical Significance

Dysfunction leads to sick sinus syndrome, inappropriate bradycardia/tachycardia.
Target site for catheter ablation in certain arrhythmias.
Vagal stimulation or ischemia in the SA nodal artery can cause sinus arrest.

5. Physiology of the Right Atrium

5.1 Role in Cardiac Cycle

The RA acts as:

Reservoir – during ventricular systole, RA fills with systemic venous blood.
Conduit – during early ventricular diastole, blood flows passively into RV.
Pump (Atrial Kick) – atrial contraction contributes 15–30% of ventricular filling, crucial during increased heart rate or diastolic dysfunction.

5.2 Atrial Pressure Waveforms

Typical RA pressure tracing shows:

a wave: atrial contraction.
c wave: bulging of tricuspid valve during isovolumetric contraction.
v wave: atrial filling during ventricular systole.
Clinical correlation: prominent a waves in tricuspid stenosis, giant v waves in tricuspid regurgitation.

6. Blood Flow Dynamics

RA receives ~75 mL of blood per beat (at rest).
Venous return is largely passive, driven by negative intrathoracic pressure and pressure gradient between systemic veins and RA.
Right atrial pressure normally ranges from 2–8 mmHg.
Clinical measurement via central venous pressure (CVP) monitoring provides insight into preload and right heart function.

7. Clinical Relevance

7.1 Arrhythmias

Atrial fibrillation and flutter often originate near the crista terminalis or SA node region.
RA is a target for cavotricuspid isthmus ablation.

7.2 Right Atrial Enlargement

Seen in pulmonary hypertension, tricuspid valve disease, chronic lung disease (cor pulmonale).
ECG: tall, peaked P waves (“P pulmonale”).
Echo: RA area > 18 cm² indicates enlargement.

7.3 Congenital Defects

Atrial septal defects (ASD): secundum, primum, sinus venosus types.
Persistent left SVC draining into RA via coronary sinus.
Ebstein anomaly (displacement of tricuspid valve leaflets).

8. Imaging & Histology

8.1 Imaging

Echocardiography: RA size, volume, pressure estimation.
Cardiac MRI/CT: detailed anatomy, RA mass, thrombus, tumor evaluation.
Fluoroscopy: important for electrophysiology procedures.

8.2 Histology

Endocardium lined by simple squamous endothelium.
Myocardium thinner than ventricular wall, arranged in crisscrossing fibers.
Contains nodal tissue (SA node) with unique electrophysiological properties.