Expansion
Generations 0 to 16 conduct air; generations 17 to 23 exchange gas
Expansion
- Conducting zone - generations 0 to 16: trachea to terminal bronchioles. No gas exchange, so this volume is the anatomical dead space
- Respiratory zone - generations 17 to 23: respiratory bronchioles, alveolar ducts and alveolar sacs
The conducting zone warms, humidifies and filters inspired air, and its ciliated epithelium with the mucus blanket forms the mucociliary escalator.
The total cross-sectional area increases enormously in the respiratory zone, from a few square centimetres at the trachea to many square metres of alveolar surface. Air velocity therefore falls almost to zero, and the final movement of gas to the alveolar membrane is by diffusion rather than bulk flow.