In the large bronchioles, the epithelial lining consists of ciliated simple columnar cells. The epithelium changes to simple cuboid cells in the small bronchioles. Goblet cells and seromucous glands become less numerous with each bronchiole division.
A ring of smooth muscle fibers surrounds the epithelium. During exhalation, these muscle fibers contract to help force air out of the bronchioles. The resulting compression causes the epithelium to fold.
The thin-walled brochioles are attached to the surrounding elastic alveoli. This connection keeps the bronchioles from collapsing during breathing movements. Because they are not needed for support, cartilage plates are characteristically absent.
