Histology

The alveolar wall has a specially adapted structure to promote gaseous exchange. It forms the boundary between millions of neigbouring alveoli. On the alveolar sides of the wall are one of several cell types:

• type I pneumocytes: covers the vast majority of the alveolar surface and key role in gaseous exchange at the alveolar-capillary complex
• type II pneumocytes: covers minimal surface area of the alveolus; role in surfactant production
• brush cells: rarely found

All of these epithelial cells are tightly bound together by zonula occludens, so minimizing the traumatic tracking of air through the wall layers or conversely, the passage of blood into the alveoli. The two layers of epithelial cells 'sandwich' an intermediate layer of pulmonary capillaries which is adapted for a minimal gaseous diffusion distance at the alveolar-capillary complex.

Alveolar macrophages exist both fixed within the alveolar wall and free on the surface of the alveolus. Infrequently, individual alveoli are connected to each other by passageways other than the alveolar sacs - the pores of Kohn.

Also within the wall between alveoli are:

• elastin fibres; contribute to the elastic recoil of the lungs
• collagens type I and III; prevent hyperdistention of lung
• occasional fibroblasts; involved in secretion of these fibres
• smooth muscle cells; located at the base of the epithelial cells. May have a role in altering the calibre of individual alveoli.