Cartilage, glands, goblet cells and cilia all disappear distally
Expansion
Four things decrease distally, and one increases
| Feature | Trachea | Bronchus | Bronchiole | Alveolus |
|---|---|---|---|---|
| Cartilage | C-shaped rings | Plates | None | None |
| Glands | Present | Present | None | None |
| Goblet cells | Many | Present | Few, then none | None |
| Cilia | Present | Present | Present, then none | None |
| Smooth muscle | Trachealis | Present | Proportionally most | None |
| Epithelium | Pseudostratified ciliated columnar | Same | Simple columnar then cuboidal | Simple squamous |
“A bronchiole is defined by having no cartilage.” That is the whole reason small airways collapse in expiration, why obstructive disease is a small airway disease, and why dynamic airway compression produces the expiratory flow limitation of COPD.
Club (Clara) cells replace goblet cells in the terminal bronchioles, secreting surfactant protein and acting as a stem cell for the bronchiolar epithelium, as well as detoxifying inhaled compounds.
Conducting versus respiratory zone: the conducting zone runs to the terminal bronchiole and constitutes the anatomical dead space; gas exchange begins at the respiratory bronchiole, which has occasional alveoli budding from its wall.
Alveolar epithelium: type I pneumocytes cover 97 per cent of the surface for gas exchange, type II produce surfactant and regenerate type I cells.