The regulation of local heat and water vapor losses along the respiratory tract is examined based on a theoretical model of respiratory air conditioning and physiological data. The theoretical model is a quasi-steady one-dimensional model descriptive of the localized process of heat and water transport within the airways. During nasal breathing the model is most sensitive to the following two parameters: 1) the gradient of blood temperature along the airway wall and 2) the nasal air space volume. Thermoregulatory control of these two factors within the primary conditioning region, the upper airway, establishes the overall respiratory heat and water loss. Upper airway thermoregulation, however, also effects the heat and water demands placed on the secondary conditioning region, the tracheobronchial airways. Similar to the upper airway, the tracheobronchial airway wall temperature varies in response to changing demands. The bronchial circulation is shown to provide a major source of heat within the first several bronchial generations where the greatest heat and water fluxes within the lung are predicted to occur. Control of the bronchial blood flow may therefore directly influence the bronchoconstrictive response in asthmatics.