Page 262 - Industrial Ventilation Design Guidebook

5.2 HUHAN RESPIRATORY TRACT PHYSIOLOGY 223

in ASL can eliminate absorbed chemicals. Diffusion through airway epithelium
into the submucosal bloodstream is an alternative removal pathway (Fig. 5.27)
that depends on lipid solubility or facilitated transport across cell membranes,

5.2.7.2 Aerosol Defense
Particle Deposition
Particles entrained in the airstream deposit along the airway as a function of
size, density, airstream velocity, and breathing frequency. Sizes of roughly spherical
or irregularly shaped particles are commonly characterized by relating the settling
101
velocity of the particle to that of an idealized spherical particle. For example, an
irregular particle which settles at the same rate as a 5 jjim spherical particle has a
mean mass aerodynamic diameter (MMAD) of 5 u,m. Since spherical particle
mass, Wp, is a function of particle diameter, d

where p = particle density, MMAD can be viewed as representing the mass and
buoyancy of a spherical particle equivalent to the randomly shaped airborne
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particle. Three basic deposition mechanisms, impaction, sedimentation, and
diffusion, act on all entrained particles, but each mechanism predominantly af-
fects specifically sized particles within a given airway region. 101

FIGURE 5.27 Diffusional pathway of deposited materials from the airstream to interstitial space.
+
Large arrows depict diffusion across the ASL, apical, and basal call membranes. Na and Cl~ passively dif-
+ +
fuse across the cellular apical surface while K* diffuses and a Na -K -2Cr~ co-transporter exchanges
these ions across the cellular basal surface. Active transport (Na+-K+ pump) also transports Na* out of
+
and K into the cell across the cellular basal surface. Water diffuses across the different cell membrane
surfaces depending upon the existing osmotic pressure gradient. Diffusion of water and salts through the
paracellular spaces between cells can also occur.