Terminal bronchioles are the air passages into the lungs. They branch into respiratory bronchioles and secrete a nonsticky surfactant to maintain the airway. Surfactant is a proteinaceous compound that maintains the airway in the smallest bronchioles and alveoli by keeping surface tension during inspiration and keeping them from collapsing during expiration.
The terminal bronchioles each branch into respiratory bronchioles, which then branch further into small alveoli. During inspiration, air is drawn down through the trachea and through primary, secondary and tertiary bronchioles to arrive in the terminal bronchioles. The terminal bronchioles are small and passage the air deep into the pulmonary tissue. The respiratory bronchioles and the alveoli have very little structural support since they are small and contain fewer layers of cells to allow for passage of ions to and from the respiratory fluid, such as oxygen.
The surfactant secreted in the terminal bronchioles helps to maintain the surface tension and structure in the respiratory bronchioles and alveoli during inspiration and expiration. The epithelium of terminal bronchioles contains club cells, which specialize in secreting proteins. These club cells secrete the proteinaceous surfactant and also secrete enzymes that neutralize any toxic substances dissolved in the respiratory fluid of the lungs.