Right atrium

The right atrium (RA) is one of the four chambers of the human heart, and is the first chamber to receive deoxygenated blood returning from the body. It plays an important role in originating and regulating the conduction of the heart.

Gross anatomy

The right atrium receives deoxygenated blood from the superior vena cava (SVC), the inferior vena cava (IVC), the coronary sinus (covered by the thebesian valve), and the thebesian veins.

It is grossly the shape of an irregular ellipsoid, with the exception of the right atrial appendage, which arises anteriorly. The right atrial appendage overlies the aortic root and the proximal right coronary artery (RCA).

• normal size (measured at end systole on four chamber view)
  • no accepted standard reference measurements
    • enlargement usually determined qualitatively
    • RA volume not routinely measured on echocardiography
  • long axis: 3.4-5.3 cm
  • short axis: 2.6-4.4 cm
  • area: 10-18 cm²
    • ​echocardiography measurement, tends to be larger on CT or MRI

The right atrium is separated from the left atrium by the interatrial septum. In cases of congenital cardiac malformations, the morphologic right atrium can be identified by the fossa ovalis in the interatrial septum.

The right atrium leads into the right ventricle through the tricuspid valve. 

One of the main anatomic landmarks of the right atrium, the crista terminalis is a muscular ridge on the anterior aspect of the chamber. The atrium is lined by pectinate muscles to the left of this crest, and these extend into the right atrial appendage. The atrial lead of an external pacemaker is frequently located in these muscles. To the right of the crista terminalis, the atrial lining is smooth and this represents the right horn of the sinus venosus.

Blood supply

• arterial supply is primarily from the right coronary artery (RCA) and several of it's branches:
  • conus artery (first branch off RCA in 55%, otherwise arises off left circumflex artery)
  • sinoatrial node artery (usually second branch off RCA in 60%)
  • acute marginal branches
• venous drainage
  • variable veins drain the atrial wall
  • tiny myocardial thebesian veins drain directly into the right ventricle

Nerve supply

The right atrium is the locatedlocation of the sinoatrial node, the heart's pacemaker. It is located subeipcardiallysubepicardially in the terminal groove, near the junction of the SVC and right atrium.

Although the sinoatrial node can generate conduction rhythms spontaneously, it is regulated by the vagus nerve (CN X) and the cardiac sympathetic plexus.

The atrioventricular node receives conduction impulses from the atrium and propagates it into the ventricular bundles. It is located between the coronary sinus ostium, the septal leaflet of the tricuspid valve, and the tendon of Todaro (which connects the thebesian and Eustachian valves).

Variant anatomy

• Eustachian valve of the IVC

Development

Develops from the right horn of the sinus venosus. Originates as two chambers, an anterior and a posterior.

Practical points

CT evaluation of the right atrium:

• the patient should be encouraged to avoid the Valsalva maneuver, which may bring unopacified blood from the IVC into the right atrium, and may mimic a thrombus
• to avoid streak artifact from dense CT contrast, a multiphasic study with 100% contrast followed by a 20-50% contrast and saline chaser should be administered

History and etymology

"Atrium" is the Latin word for "court", referring to the central area in a Roman house from which one could enter various chambers. It was entered through the "ostium" of a Roman house.

Related pathology

• atrial septal defects
  • ostium primum
  • ostium secundum
  • sinus venosus
  • patent foramen ovale
• tricuspid stenosis
• tricuspid regurgitation
• right atrial enlargement
• cor pulmonale
• Ebstein anomaly
• cor triatriatum (dexter)
• atrial infarction
• lipomatous hypertrophy of the interatrial septum