1. The airways branch roughly 16 to 17 times before alveolar sacs are encountered.
The airways branch roughly 16 to 17 times before alveolar sacs are encountered. The surface area of the human airways averages ∼2 to 3 m2, compared with roughly 100 m2 for the alveolar surface. In the upper airways, the inertia of the larger particles causes them to break free of the streamlines of the flow and collide with a wall to be deposited. As impaction clears these larger particles in the upper airways, slightly smaller particles are filtered out of the airstream in the middle airways by gravitational sedimentation. Finally, for very small particles, particle motion is determined by Brownian diffusion, which accounts for the dominant mechanism of deposition in the alveolar region. (Reprinted from reference 46 by permission from Macmillan Publishers Ltd.)‏
Eric Wenzler et al. Clin. Microbiol. Rev. 2016; doi: /CMR