Interalveolar septum

At the level of the alveoli, where gas exchange occurs, the wall of the passage has been reduced to minimal layers and minimal thickness: a simple squamous epithelium resting on a thin connective tissue core that houses a vast capillary plexus. The thinness of the barrier separating incoming air from capillary blood facilitates gaseous exchange. 1000x

The walls of adjacent alveoli abut to form interalveolar septa. Each septum is lined by a simple squamous epithelium; connective tissue forms the core of the septum. Numerous capillaries bulge from the septa into the lumen of each alveolus.

The epithelium lining the alveolus consists of an extremely thin simple squamous epithelium (arrows) along with cuboidal secretory cells (squares). Most of the alveolar surface is covered by the squamous cells.

The surface of the alveolus is coated by surfactant, a thin lipoprotein layer that reduces surface tension in the alveolus, thus aiding inspiration. The surfactant is produced by the cuboidal cells in the lining epithelium.

The alveolar epithelium rests on a thin core of connective tissue consisting of elastic and reticular fibers and various connective tissue cells. An extensive capillary network is present in the core.

Due to the thinness of the septum, capillaries bulge into the alveolar lumen.

Each capillary is lined by a simple squamous epithelium, the endothelium.

The barrier through which oxygen and carbon dioxide must pass consists of the simple squamous epithelium lining the alveolus, the capillary endothelium, and the fused basal laminae of each epithelium. This is often referred to as the blood-air-barrier.