Left atrium

Last revised by Raymond Chieng on 12 Oct 2023

The left atrium (LA) (plural: atria) is one of the four chambers of the heart. It receives oxygenated blood from the pulmonary circulation that is then delivered to the left ventricle and then into the systemic circulation.

The left atrium is grossly cuboidal. It is the most superior and posterior portion of the heart and is obscured anteriorly by the pulmonary trunk and ascending aorta. It lies mostly to the right of the left ventricle.

The left atrium is separated from the right atrium by the interatrial septum, with a small depression, the fossa ovalis, at the site of the closed foramen ovale. It is smaller by volume than the right atrium but with thicker walls.

The left atrial appendage is long and narrow, curving anteriorly from the left around the pulmonary trunk, overlying part of the left coronary artery. Its inner surface is lined by the small pectinate muscles, unlike the main cavity which has smooth walls 7.

Blood enters on either side via four pulmonary veins, the left and right superior and inferior, and exits via the left atrioventricular opening through the mitral valve, into the left ventricle. The mitral valve is bicuspid with anterior and posterior leaflets supported by the left atrium.

See innervation of the heart.

  • accessory left atrial appendage: common variant in which small diverticular structure is seen on the superior right side

  • left atrial diverticulum: a pouch-like diverticulum, differentiated from accessory atrial appendage by this smooth shape suggesting a lack of pectinate muscles

  • coumadin ridge: band-like embryological remnant between the left superior pulmonary vein and the left atrial appendage

On contrast-enhanced chest CT and cardiac MRI, the left atrium when measured on axial slices can be considered enlarged when the anteroposterior diameter is ≥50 mm (male) and ≥45 mm (female) 6.

In the fetal heart, the left and right atria communicate via the foramen ovale in the interatrial septum. Both atria develop from a single primitive atrium. The only remnant of this in the left atrium is the atrial appendage. The smooth-walled main cavity of the left atrium develops from the pulmonary veins. See development of the heart.

"Atrium" is the Latin word for an entrance hall, 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.

The interatrial septum may be difficult to identify on CT. This is especially true of the fossa ovalis, being even thinner, which may be unidentifiable and mistaken as an atrial septal defect.

The junction of the left atrial appendage and left superior pulmonary vein forms a muscular ridge. This ridge varies in size and may be large and pronounced, and thus mistaken for a pedunculated mass or thrombus in the left lateral wall.

The left atrium lies directly in front of the esophagus and is, therefore, an appropriate window for transesophageal echocardiography.

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