The reasoning: structure dictates defence
Weibel's symmetric model: generations 0 (trachea) to ~16 (terminal bronchiole) form the conducting zone (anatomical dead space ~150 mL); ~17–23 (respiratory bronchioles → alveolar ducts → sacs) form the respiratory zone (acinus). Cartilage and submucosal glands disappear at the bronchiolar level; smooth muscle continues distally. Epithelium transitions from pseudostratified ciliated columnar (basal, ciliated, goblet cells) to simple cuboidal with club cells in bronchioles, then AT1 (≈ 95% of alveolar surface) and AT2 cells (surfactant, progenitor) in alveoli. Total cross‑sectional area rises steeply distally, so gas velocity falls and diffusion takes over at the acinus.