alveolar duct

**Position in the airway tree:** Trachea → bronchi → bronchioles → terminal bronchioles (end of the conducting zone, ~generation 16) → respiratory bronchioles (start of the respiratory zone, ~generations 17–19) → alveolar ducts (~generations 20–22) → alveolar sacs (~generation 23) → alveoli.

**Histology (high-yield):** - The wall is mostly alveolar openings, so the duct looks like a tube lined by a series of alveoli. - The epithelium is simple squamous (type I pneumocytes) with some type II pneumocytes within the alveoli. Cuboidal cells may appear near the proximal end. - Smooth muscle and elastic and collagen fibers are present only as small "knobs" or rings between adjacent alveolar openings. These knobs are the classic histologic landmark. - Cartilage, glands and goblet cells are absent. There are no cilia in the alveoli, and Club (Clara) cells are limited to the bronchioles. - Alveolar ducts end in alveolar sacs, which are clusters of alveoli sharing a common opening (atrium).

**Physiology:** - It is part of the respiratory zone, where gas exchange occurs (conducting zone = anatomic dead space). - Cross-sectional area is very large at this level, so airflow velocity is very low and gas movement occurs mainly by diffusion rather than bulk flow. - Airway resistance is highest in the medium-sized bronchi and lowest in the distal airways because of their large total cross-sectional area. - Elastic fibers provide recoil. Loss of elastic tissue and alveolar wall destruction leads to emphysema.

**Clinical correlations:** - **Emphysema:** Destruction of alveolar walls distal to the terminal bronchiole. *Centriacinar* (smoking; affects respiratory bronchioles, upper lobes) and *panacinar* (α1-antitrypsin deficiency; entire acinus including alveolar ducts, lower lobes) are the two main types. Loss of elastic recoil causes increased compliance, air trapping and a barrel chest. - **Surfactant deficiency / neonatal RDS:** Type II pneumocytes (which make surfactant) are present in the alveoli lining the ducts and sacs. Decreased surfactant causes alveolar collapse and a low lecithin:sphingomyelin ratio (<1.5–2). - **Pneumoconioses and interstitial lung disease:** Dust deposits and fibrosis affect the respiratory bronchioles and alveolar ducts, which are the distal sites of particle deposition (e.g., coal workers' pneumoconiosis with centriacinar macules). - **Acinus:** The functional unit distal to the terminal bronchiole (respiratory bronchioles, alveolar ducts, alveolar sacs and alveoli).

**Memory aid:** Terminal bronchiole = the last "conducting" airway. After that, anything with alveoli in its wall (respiratory bronchiole, alveolar duct, alveolar sac) takes part in gas exchange.