Type I alveolar cells: (a) allow rapid diffusion of gases through their membranes. (b) secrete a chemical known as surfactant. (c) are phagocytic. (d) A, B, and C. (e) None of the above. Solution (a) allow rapid diffusion of gases through their membranes. Type I alveolar cells are squamous which gives more surface area to each cell and cover approximately 90–95% of the alveolar surface. Type I cells are involved in the process of gas exchange between the alveoli and blood. These cells are extremely thin.These cells need to be so thin to be readily permeable for enabling an easy gas exchange between the alveoli and the blood..