A pseudostratified epithelium is a type of epithelium that, though comprising only a single layer of cells, has its cell nuclei positioned in a manner suggestive of stratified epithelia. As it rarely occurs as squamous or cuboidal epithelia, it is usually considered synonymous with the term pseudostratified columnar epithelium.
The alveoli are sites where air is exchanged in the lungs. Simple squamous epithelial cells in the alveoli allow oxygen from the air to enter the blood in the capillaries of the lung. Carbon dioxide, a waste product, passes across the epithelium of the alveoli to be removed from the body.
Although this epithelium is referred to as squamous, many cells within the layers may not be flattened; this is due to the convention of naming epithelia according to the cell type at the surface. In the deeper layers, the cells may be columnar or cuboidal.