Overview Airway lining pseudostratified ciliated columnar cells extend to the bronchioles mucus secretions are swept out of the lungs toward the mouth by ciliated cells smoking causes these cells to undergo metaplasia replaced by stratified squamous simple ciliated columnar epithelium extends to the terminal bronchioles ciliated cuboidal cells extend to the respiratory bronchioles alveolar sac are composed of pneumocytes macrophages clear debris in alveoli goblet cells extend to the larger bronchioles but stop before the terminal bronchioles Type I pneumocytes 97% of alveolar surfaces line the alveoli squamous thin for optimal gas diffusion form blood-gas barrier with pulmonary capillary endothelial cells Type II pneumocytes 3% of alveolar surfaces secrete pulmonary surfactant within lamellar bodies dipalmitoyl phosphatidylcholine lowers the alveolar surface tension cuboidal and clustered serve as precursors to type I cells and other type II cells proliferate during lung damage Club (Clara) cells nonciliated columnar with secretory granules secrete component of surfactant degrade toxins act as reserve cells A lecithin-to-sphingomyelin ratio of > 2.0 in amniotic fluid is indicative of fetal lung maturity