Page 258 - Industrial Ventilation Design Guidebook

5,2 HUMAN RESPIRATORY TRACT PHYSIOLOGY 2 I 9

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bronchial circulation, and increasing airway smooth muscle tension. " ^ Un-
der pathological conditions, diminished conditioning may also increase mucus
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thickness, ' which in extreme cases causes increased airway resistance by re
ducing airway cross-sectional area and increasing shear stress at the air/mucus
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interface. In addition to effects on the conducting airways, alveolar O 2 and
CO 7 transport could be hampered if air has not been warmed to body temper-
ature (37 °C) and fully humidified by the time it reaches the alveoli.
The importance of respiratory heat and water losses is not confined to
the respiratory structures. Inspiration of cold, hot, or dry air poses the po-
tential threats of thermal injury or desiccation to the airway
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epithelium ' ' ' ' and is a challenge to whole-body thermoregulation.
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Under certain conditions, such as hyperbaria, ' airway heat losses can
account for a considerable percentage of total body heat production (in
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some cases > 100%). Normally these threats are ameliorated by rapid
moderation of inspired air temperature and humidity by exchanging heat
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and water vapor between the mucus and airstream in the upper airway. '
Recovering much of the heat and water vapor contained in expired air
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minimizes heat and water losses to the ambient environment and aids in
whole-body thermoregulation.
Heat and water vapor transport can also lead to respiratory impairment, in-
fection, and injury through thermal and osmotic stresses occurring at the mu-
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cosal epithelium. ' These stresses cause changes in mucus osmolarity, pH,
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ciliary activity, and cellular transport, ' resulting in altered mucosal
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thickness ' and impaired airway defenses. Normal breathing allows microor-
ganisms, pollutant gases, and particulate matter to contact the mucus coating
(comprised of mucus gel and periciliary fluid) atop the apical surface of respira-
tory epithelium. A complex system of chemical, immunological, and mechanical
defense mechanisms protects the respiratory epithelium and alveoli from poten-
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tial diseases or injury caused by noxious airstream components. Aside from
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chemical neutralization of pollutants in the airstream ' and physical defenses
such as bronchoconstriction, coughing, and particle impaction caused by airway
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morphology, ' " the defense of the airway depends on the physical and
chemical properties of airway mucus (e.g., chemical detoxification reactions
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with proteins ) and the ciliary mechanism which moves it toward the epiglottis
(mucociliary escalator). Inspiring cold dry air can impede mucociliary transport,
reducing mucus velocity and increasing the risk of airway disease or injury.
Airway deposition patterns of inspired hygroscopic particles are also af-
fected by airway heat and water vapor exchange. Inspired particles passing
from a relatively dry ambient environment into the fully saturated airway
quickly adsorb water from the surrounding airstream. Water vapor adsorption
at the particle surface increases hygroscopic particle mass m p as a function of
particle diameter d g and the water vapor concentration gradient between the
bulk fluid, £«,, and particle surface, CQ, according to

where D w— diffusion coefficient of water vapor in air and C w = slip correction
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factor = f(d g, D t^ T). In addition, d^ can be determined from nip and particle
density, p,