Cardiac Atria: Anatomy, Function, and Electrical Role

The atria are the two upper chambers of the heart that serve as receiving chambers and contribute to ventricular filling and cardiac rhythm.

Anatomic Overview

Right Atrium (RA)

• Receives: Systemic venous return (~5 L/min)
• Inflow: Superior vena cava (SVC — upper body), Inferior vena cava (IVC — lower body), Coronary sinus (cardiac venous return)
• Key structures: Crista terminalis, fossa ovalis (foramen ovale remnant), SA node (sinus node), Eustachian valve (IVC), Thebesian valve (coronary sinus)
• Outflow: Tricuspid valve → Right ventricle

Left Atrium (LA)

• Receives: Oxygenated pulmonary venous return
• Inflow: Four pulmonary veins (2 right, 2 left)
• Key structures: Left atrial appendage (thrombus source in AF), mitral annulus
• Outflow: Mitral valve → Left ventricle

Physiologic Functions

1. Reservoir: Collect venous return during ventricular systole
2. Conduit: Passive emptying during early ventricular diastole
3. Booster pump: Atrial contraction ("atrial kick") contributes 15–30% of ventricular filling (critical in diastolic dysfunction)

Electrical Role

• SA Node (RA): Primary pacemaker; spontaneous depolarization → sinus rhythm (60–100 bpm)
• AV Node (interatrial septum): Delays impulse → allows atrial contraction before ventricular
• Bachmann's bundle: Interatrial conduction → synchronous atrial activation
• Atrial arrhythmias: AFib, flutter, tachycardia originate from atrial substrate (PVs, RA, LA)

Clinical Relevance

• Atrial fibrillation: Loss of atrial kick → 15–30% CO reduction; thrombus in LAA → stroke risk
• Atrial enlargement: LA size predicts AF, HF, mortality
• Congenital: ASD, PFO, TAPVR alter atrial hemodynamics

The atria are not passive antechambers—they are dynamic, electrically active structures integral to cardiac output and rhythm stability.