In cross-section, the bronchus wall appears similar to the trachea.
A layer of respiratory mucosa lines the lumenal surface. Mucus-secreting goblet cells are present in the epithelium. However, they are less numerous than in the trachea.
Deep to the mucosa is a broken ring of smooth muscle fibers. During exhalation, these fibers contract to help force air out of the respiratory tubes. The fibers relax again during inhalation so air can be drawn deeper into the lungs.
Plates of hyaline cartilage are present in the tissue around the muscle fibers. They support the wall and help keep it from collapsing. In the secondary and tertiary bronchi, the plates are thinner and less numerous.
