Which cells make up most of the alveolar surface area and are highly permeable to gases?

1. The alveolar wall carries three major cell types: type I cells, type II cells and alveolar macrophages. 2. Type I alveolar cells are squamous epithelial cells that constitute up to 97 percent of the alveolar surface area. 3. They are highly permeable to gases, which is exactly what an exchange surface needs. 4. Type II cells are interspersed among them and secrete surfactant rather than forming the bulk of the surface. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 22 "The Respiratory System", section 22.1 Organs and Structures of the Respiratory System_