trachea divides into two chief bronchi one for each lung, entering at the hilus, the bronchus divides into smaller bronchi which gives rise to several orders of bronchioles.
The general structure consists of a mucosa, muscularis, a fibro-elastic membrane which encloses a cartilage and loose collagenous peribronchial layer.
Gradual changes in structure occur, as the main bronchus, divides into smaller bronchi, several order of bronchioles till the segment called terminal bronchiole is reached.
- These changes consist of the following.
- The mucosa is lined by pseudo-stratified columnar cilated epithelium, in the larger bronchi.
- The epithelium gradually decreases in height and the goblet cells become fewer.
- The epithelium becomes simple columnar ciliated with no goblet cells in the terminal bronchiole.
- The glands in the submucosa progressively decrease and finally disappear.
- There is a relative increase of smooth muscle till the terminal bronchiole.
- The hyaline cartilage does not occur in the form of rings as in trachea but as isolated plates in bronchi, which also disappear, in bronchioles.
Bronchi
- Mucosa: Pseudostratified columnar ciliated epithelium with numerous goblet cells. In submucosa mucous glands are present. Plain muscle lies inner to cartilage plates and forms a continuous cuticular layer (unlike in the trachea). Hyaline cartilage occurs only in the form of isolated plates.
- Bronchioles (small unit) Mucosa simple columnar ciliated epithelium with Goblet cells. No glands occur in the submucosa. Smooth muscle forms a continuous cirular layer. Cartilage plates are absent.
- Terminal Bronchioles: The mucosa is lined by simple columnar ciliated epithelium. No goblet cells are present and no glands occur below the mucosa. A relatively thicker smooth muscle is present, in the wall but cartilage is absent. The terminal bronchiole divides into two or more respiratory bronchioles.
