Body of left atrium

The body of the left atrium is the main chamber of the left atrium, excluding the left atrial appendage and (for some sources) the vestibular component. It consists of a posterosuperior venous component (Sinus of pulmonary veins), which receives the four pulmonary veins, and an anteroinferior vestibular component that surrounds the mitral valve orifice. The interatrial septum separates the left atrial body from the right atrium. The venous component forms a prominent atrial dome, and the walls of the left atrial body are muscular and non-uniform in thickness, typically thicker than those of the right atrium. 

The left atrial appendage is anatomically distinct from the body, with its own orifice and trabeculated structure, and is not included in measurements of left atrial volume or endocardial border tracing.

Embryologically, the body of the left atrium arises from the incorporation and growth of the pulmonary veins, resulting in a smooth-walled structure that is histologically identical to the vessel wall covered by extrapericardially differentiated myocardium, that explains the term "Smooth part of left atrium" used in Terminologia Anatomica 2 for the body of the left atrium.