Page 260 - Industrial Ventilation Design Guidebook

5,2 HUMAN RESPIRATORY TRACT PHYSIOLOGY 22 1

lization of NH 3 in ammo acid synthesis. Ureolysis appears to be very sensitive to
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pH; NH 3 production increases as salivary pH is reduced from 7.0 to 6.0 but
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decreases significantly when pH is lowered to approximately pH 2.5. A sali-
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vary pH of 2.5, however, only temporarily depresses NH 3 production since
NH 3 diffusing from the bloodstream may neutralize acids responsible for re
duced oral cavity pH and slowly increase oral pH. Ureolysis may increase rap-
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idly at some pH threshold, perhaps near pH 5.5, because of the steady supply
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of salivary urea. Oral pH continues to increase as NH 3 is generated, with
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peak NH 3 production thought to occur near an oral pH of 6.0. Salivary
HCOg may act to buffer increases in oral pH and thus maintain NH 3 produc-
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tion rates. Therefore, an increase in salivary flow will not only increase the
availability of urea to oral bacteria but also help maintain oral conditions ad-
vantageous for NH 3 production. Theories regarding in vivo regulation of oral
NH 3 production are speculative since the bulk of data was obtained from in
vitro studies of salivary sediments and dental plaque samples; greater knowl-
edge of in vivo interaction between oral cavity NH 3 production, pH, and saliva
is needed.
Fasting combined with poor oral hygiene results in an elevated dental
%
plaque pH ( ~7.6), suggestive of active ureolysis. Whether fasting or poor
oral hygiene is responsible for the higher pH is unclear. Carbohydrates in the
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mouth lower dental plaque pH, while glucose, in particular, buffers oral
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pH thereby inhibiting NH 3 production. The formation of NH 3 appears to
be inhibited by glucose for two other reasons: (a) it is preferentially used for
bacterial energy production in place of proteins and peptides, and (b) its pres-
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ence favors acid-producing bacteria that scavenge NH 3. Oral food residues
with a high protein content should serve as a rich substrate for oral NH •, pro-
duction through bacterial deamination.

5,2.7 Respiratory Defense Mechanisms
Breathing exposes a large body surface area to attack by noxious materials or
pathogens present in the ambient atmosphere. A complex series of defense
mechanisms, including physical removal, chemical neutralization, and immu-
nological response, protect against biological, chemical, or mechanical injury.
Physically removing toxins or pathogens from the airstream by coughing,
sneezing, movement along the mucociliary escalator, or phagocytosis in the al-
veoli reduces exposure concentrations in vulnerable airway regions. Airborne
toxins can be neutralized by endogenous NH 3, while deposited materials are
diluted, buffered, and neutralized by periciliary fluid and mucus gel. Immuno-
globins and enzymes present in periciliary fluid and along alveolar surfaces
can eliminate deposited pathogens that have not been physically removed.

5.2.7.1 Vapor Phase Neutralization
In addition to typical atmospheric or metabolic constituents (N 2 , O?, H^O,
and CO 2), breathing transports chemical species in the form of vapors and par-
ticulates along the respiratory tract. While O 2 and CO 2 gas exchange occurs
solely in the lung parenchyma, air/blood exchange of other species can and does
occur throughout the airway. The principal airway absorption sites of gases such
as O 3 and SO 2 are largely determined by their watenair partition coefficient (the
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concentration ratio at equilibrium) and water solubility. Highly water-soluble