Anatomy of the lungs and airways

The branching tree - 23 generations, trachea to alveolus

Zone	Generations	Structure	Function
Conducting	0-16	Trachea -> bronchi -> bronchioles -> terminal bronchioles	No gas exchange - the anatomical dead space (~150 mL, ~2 mL/kg)
Transitional/respiratory	17-23	Respiratory bronchioles -> alveolar ducts -> alveolar sacs	Gas exchange

Cartilage is present in the trachea and bronchi (>1 mm diameter) and absent beyond the ~8th generation
- Bronchioles (<1 mm) have no cartilage and depend entirely on radial traction from the surrounding parenchyma for patency
- -> loss of elastic recoil (emphysema, ageing) -> small airway collapse on expiration -> air trapping. This is why emphysema causes airflow obstruction without any airway disease.
Bronchus = cartilage + submucosal glands; bronchiole = neither

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