Right atrium

Atrium dextrum

Also known as: RA

The right atrium receives deoxygenated blood from the SVC, IVC and coronary sinus and passes it through the tricuspid valve to the right ventricle. It contains the sinoatrial and atrioventricular nodes.

Overview

Mean pressure≈2–6 mmHg

Wall thickness≈2 mm

The right atrium forms the right border of the heart. It has two parts with different embryological origins. The smooth-walled posterior sinus venarum comes from the right horn of the sinus venosus and receives the venae cavae. The rough anterior part, lined with pectinate muscles, comes from the primitive atrium and extends into the ear-shaped right auricle. Inside, a muscular ridge called the crista terminalis separates the two parts. Outside, a shallow groove, the sulcus terminalis, marks the same boundary. The SA node lies at the top of the crista, where it meets the SVC.

Openings. The SVC enters from above and has no valve. The IVC enters from below, guarded by the rudimentary valve of the IVC (Eustachian valve). In the fetus this valve directed oxygen-rich IVC blood across the foramen ovale. The coronary sinus opens between the IVC and the tricuspid orifice, guarded by the Thebesian valve. Small anterior cardiac veins and venae cordis minimae also drain into the chamber. The tricuspid orifice lies anteromedially.

Septal wall. The posteromedial wall is the interatrial septum. It carries the fossa ovalis, an oval depression with a raised rim called the limbus.

Triangle of Koch. This triangle is bounded by the tendon of Todaro, the attachment of the septal tricuspid leaflet and the opening of the coronary sinus. The AV node lies at its apex.

Location

Right side of the heart, forming the right border on a frontal chest radiograph and the right pulmonary surface. It lies in front of and to the right of the left atrium.

Parts & subdivisions

Sinus venarum

Smooth posterior part that receives the SVC, IVC and coronary sinus.

Crista terminalis

Muscular ridge between the smooth and pectinate parts, running from the SVC to the IVC. It is a preferential conduction route.

Pectinate muscles and right auricle

Comb-like parallel muscular ridges lining the anterior wall and the auricle.

Fossa ovalis and limbus

Remnant of the fetal foramen ovale on the septal wall.

Opening of the coronary sinus

Lies between the IVC and the tricuspid orifice and is guarded by the Thebesian valve.

Triangle of Koch

Landmark for locating the AV node, which must be avoided during ablation.

Sinoatrial node

The pacemaker, located where the SVC meets the crista terminalis.

Anatomy

Arterial supply

Right coronary artery: atrial branches and, in about 60% of people, the SA nodal artery.

Venous drainage

Small cardiac veins and venae cordis minimae drain directly into the chamber.

Relations

Laterally: the right lung and pleura, with the right phrenic nerve running over the pericardium. Above and below: the SVC and IVC. Medially and in front: the aortic root. Behind: the right pulmonary veins.

Function

Receives systemic and coronary venous blood.

Acts as a reservoir during ventricular systole and as a conduit in early diastole.

Completes right ventricular filling with its contraction (the atrial kick).

Houses the pacemaker (SA node) and the AV node.

Releases ANP and triggers the Bainbridge reflex when stretched.

Physiology

Mean right atrial pressure is about 2–6 mmHg and equals the central venous pressure. The pressure waveform has three positive waves. The a wave is atrial contraction. The c wave is the tricuspid valve bulging back at the start of ventricular systole. The v wave is venous filling against the closed tricuspid valve. Between them are two descents. The x descent reflects atrial relaxation and the downward movement of the valve ring. The y descent follows opening of the tricuspid valve. The same waves are visible in the jugular venous pulse.

Clinical significance

Raised right atrial pressure

A raised JVP reflects high right atrial pressure. Causes include right heart failure, tricuspid regurgitation (which produces giant v waves) and cardiac tamponade. Cannon a waves occur in complete heart block, when the atrium contracts against a closed tricuspid valve.

Typical atrial flutter

In typical atrial flutter, an electrical impulse circles the tricuspid annulus in a large re-entrant loop. This produces sawtooth flutter waves at about 300/min. Ablating the cavotricuspid isthmus, the strip of tissue between the IVC and the tricuspid annulus, cures it.