The alveoli walls consist of two types of epithelial cells.

1.

Type I alveolar cells consisting mainly of a single layer of squamous cells

2.

Type II alveolar cells small, cuboidal secretory cells scattered around the alveolus. These cells secrete a surfactant, which helps keep alveoli inflated.

Apart from the cells that form part of the alveolar walls, the alveoli also contain macrophages that have migrated from the blood stream. These phagocyctic cells either adhere to the alveolar wall or roam freely in the lumen. Their function is to ingest and destroy microorganisms and other foreign particles within the alveoli.

The walls of the alveoli and of the capillaries make up what is known as the alveolar-capillary membrane. This membrane is the site of gas exchange and is made up of the following layers:

1.

Alveolar wall (Type I and Type II alveolar cells)

2.

Alveolar epithelial basement membrane

3.

Capillary basement membrane

4.

Capillary endothelial cells