Page 257 - Industrial Ventilation Design Guidebook

2 I 8 CHAPTER 5 PHYSIOLOGICAL AND TOXICOLOGICAL CONSIDERATIONS

is cooler than this, creating a radial temperature gradient, such that the air within
the airway lumen is cooler than the walls (Fig 5.25). A radial water vapor con-
centration gradient also exists because air at the air-mucus interface (airway
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wall) is fully saturated, while inspired air has a lower absolute humidity due to
its being at a lower temperature than the airway wall.
Under most circumstances, passage of relatively cool inspiratory air along
the airway results in convective and evaporative cooling of the mucosa while
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warming and humidifying the inspired air. ' Airflow patterns caused by
convoluted upper airway morphology augment heat and water vapor trans-
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port. " Radial temperature gradients can persist at least as far as the carina
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during oral breathing of room air, - causing heat and water vapor exchange
to occur for much of the length of the upper airway. At the end of inhalation,
longitudinal temperature and water vapor concentration gradients exist along
the airway (Fig. 5,25). Air reaching the most distal airway regions (beyond
about the fourteenth bronchial generation) is believed to be fully conditioned
(37 °C, 100% humidity) during normal breathing. Exhalation causes warm
air originating in the distal airway to pass over airway walls in proximal air-
way segments, which had been cooled during inspiration and are normally
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maintained below body core temperature. The resulting temperature gradi-
ent causes airstream water vapor to condense and the airstream to lose heat to
the airway walls. This acts to minimize net heat and water losses. 48
Effectiveness of the conditioning process is dependent upon respiratory
tract geometry, ambient air temperature (T amj,) and humidity (C amj,), inspira-
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tory and expiratory flow rates and volumes, mucus temperature (?,„), air-
way wall blood temperature (T blood), and flow rate in the submucosal capillary
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bed. " These variables interact and their effects are interdependent. For ex-
ample, airway geometry plays a major role in conditioning since the rate of
heat exchange between the wall and airstream, q is given by

where h = heat transfer coefficient, A s — airway wall surface area, and
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AT = temperature difference between the airstream and wall. Water vapor ex-
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change is analogous to heat exchange and thus is also a function of A s. In addi-
tion, both mean gas velocity, u and residence time, t w , are dependent on airway
geometry since t w = f(u) , u = f(A) , and conduit geometry directly affects the
generation of flow disturbances and subsequent development of turbulent flow,
5.2.5.2 Role of Airway Heat and Water
Vapor Exchange in Disease and Injury
Exchange of heat and water vapor in the respiratory tract can significantly
influence airway patency, alveolar gas transport, and whole body homeostasis,
such as seen with cold- or exercise-induced bronchospasms. Maintaining air-
way patency is important in reducing airway resistance, maximizing inspira-
tory volume, and minimizing the work of breathing. The mechanism by which
heat and water vapor exchange influences airway resistance has been widely
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debated " but probably depends on both airway mucosa heat and water
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losses. ' It has been suggested that alterations in the conditioning of inspired
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and expired air can lead to increased total airway resistance ' ' ' by causing
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increased nasal blood flow, ' altering vascular tone and permeability in the