Bronchi are tube-like structures that allow air to be transported from the surrounding atmosphere to the lung. The air is filtered from potentially harmful particles while passing through the bronchi. The respiratory system (airways) consists of the trachea (essentially a large bronchus) that branch off into smaller and smaller bronchi before reaching the most distal lung alveoli, where oxygen can be transferred to the blood.

The bronchus histologically consists of the following five layers:

  • The mucosa, lining the inside of the bronchus. The mucosa, in turn, can be divided into respiratory epithelium, the most superficial layer, and lamina propria, the tissue right beneath the respiratory epithelium
  • The muscularis mucosa, a smooth muscle layer under the mucosa.
  • The submucosa, a connective tissue layer with seromucous glands.
  • The cartilaginous layer, a layer of cartilage plates located beneath the submucosa.
  • The adventitia, the deepest layer separating the bronchus from surrounding tissues.