Page 261 - Industrial Ventilation Design Guidebook

222 CHAPTER 5 PHYSIOLOGICAL AND TOXICOLOGiCAL CONSIDERATIONS

chemical species with high watenair partition coefficients are generally absorbed
in the extrathoracic airways, while less soluble species pass beyond the extratho-
racic airways in relatively high concentrations (see Section 5.3).
Concentration gradients provide the driving force for gaseous chemical spe-
cies diffusion between the luminal gas mixture and ASL. Factors that alter this
gradient, such as local airstream concentration, chemical reactivity, lipid solubil-
ity, and ASL metabolism, modulate local absorption or reentrainment into the
airstream. Local airstream/ASL concentration gradients drive diffusion into or
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out of ASL along a given airway length. ' As the inspiratory air passes along
the airway and comes into contact with previously unexposed ASL, chemical
species follow the concentration gradient and diffuse into ASL. The leading edge
of the inspiratory wave becomes increasingly depleted of the diffusing species,
increasing proximal ASL concentrations while reducing airstream concentra-
tions downstream. Consequently, ASL absorption decreases in more distal air-
ways. Reentrainment can occur during expiration if concentration gradients are
reversed, i.e., tracheobronchial and extrathoracic ASL concentrations exceed
those found in gases flowing outward from the lung parenchyma.
The rate at which an absorbed chemical species is removed from the ASL
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determines whether reentrainment occurs during a breathing cycle. Slow re-
moval rates relative to the breathing cycle allow the concentrations in the ASL
to be higher than in the expiratory airstream. Figure 5.26 shows processes that
diminish the ASL concentration of absorbed chemical species. Metabolic pro-
cesses or interactions with ions and other chemically reactive substances found

FIGURE 5.26 Various chemical and physical mechanisms which can affect ASL chemical con-
centration during breathing. Dilution due to transepithefial water exchange depends on the osmotic
pressure gradient between periciliary and interstitial fluid.