Conducting zone
Trachea → terminal bronchioles, ~16 generations
Convection: bulk flow and streaming.
No exchange at all — this is plumbing.
Respiratory zone
Diffusion; flow velocity ≈ 0.
O₂ and CO₂ cross the membrane here.
Flow only has to reach the doorstep: across one alveolus, t ≈ x²/D ≈ 2.5 ms.