The walls of the alveoli are composed of two types of cells, type I and type II. What is the function of type I?
Type I alveolar cells comprise the major gas exchange surface of the alveolus and are integral to the maintenance of the permeability barrier function of the alveolar membrane.
Type I alveolar cells are squamous epithelial cells, covering about 90-95% of the alveolar surface. They are thin walled cells and readily permeable, thus enabling an easy exchange between the alveoli and the blood.
The type I alveolar cell is a complex branched cell with multiple cytoplasmic plates that are greatly attenuated and relatively devoid of organelles. These plates represent the gas exchange surface in the alveolus. The thin layer of cytoplasm contains pinocytotic vesicles which may play a role in the removal of small particulate contamination from the outer surface. In addition to desmodomes all type I alveolar cells have occluding junctions that prevent the leakage of tissue fluids into the alveolar air space.