Page 256 - Industrial Ventilation Design Guidebook

5.2 HUMAN RESPIRATORY TRACT PHYSIOLOGY 2 I 7

5.2.5 Airway Heat and Water Vapor Transport
5.2.5.1 Longitudinal and Radial Temperature/Humidity Gradients
Air passing through the respiratory tract must be properly conditioned
(warmed and humidified) in order to optimize alveolar O 2 and CO 2 transport
and minimize heat and water losses from the body. Respiratory air conditioning,
as shown in Fig. 5.25, occurs as the airstream passes over the airway mucosal
surfaces and results in both spatial and temporal humidity and temperature
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changes during each phase of respiration. ' Submucosal blood temperatures
(Ty ood) are thought to be cooler in the extrathoracic airways and gradually warm
along the length of the conducting airways until body core temperature (T core,
roughly 37 °C) is achieved within the bronchi. This longitudinal temperature
gradient exists because ambient air temperatures are generally lower than T core.
Inspired air extracts heat and water from airway walls as it passes through the lu-
men, warming the airstream and cooling the wall. The radial temperature gradi-
ent lessens as the ever-warmer airstream moves along the airway, causing
gradually less heat extraction from the wall until eventually no heat is ex-
changed. The process is reversed during expiration. At the onset of inspiration,
the walls of the airway are their warmest and approach end-expiratory airstream
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temperatures throughout most of the respiratory tract. ' Typically, inspired air

FIGURE 5.25 The relationship between the inspirator/ and expiratory airstream front boundary and
air, T g, and wall, 7~ m, temperatures as a function of nondimensional distance from the nares. Solid lines shown
on the airway picture indicate respiratory fronts and arrows depict the direction of airflow. Qualitative tem-
peratures indicate relative temperature gradients across front boundaries. The graph depicts airstream tem-
peratures as a function of nondimensional distance along the airway while breathing normal room air.
Nondimensional distance is distance from the nares (x) divided by overall airway length (L).