488 RESPIRATION 



and depth of the respiratory movements, the intensity of the metab- 

 oHsm, and other factors. Under ordinary conditions, however, 

 air of 20° C. is warmed to the temperature of the body, or nearly so, 

 while, at lower temperatures, the rise as such may be greater but does 

 not reach 37° C. At 6.3° C, for example, the inspired air is heated to 

 29.8° C. The greatest heat absorption takes place in the deeper 

 respiratory channels, while the difference in the temperature of the 

 outside air and that in the lower portion of the trachea amounts to 

 only a few degrees centigrade. It is evident, therefore, that this loss 

 of body-heat is effected very largely through the blood of the pulmonary 

 circuit and adjoining venous trunks. This fact is made use of in the 

 open air treatment of respiratory diseases for purposes of lowering 

 the body-temperature. Ordinarily, of course, the respiratory tract 

 of man does not play an important part in heat dissipation, but 

 some animals, and especially those possessing a thick covering of 

 hair, are almost wholly dependent upon this channel for the regula- 

 tion of their body-temperature. 



In consequence of this absorption of heat, the intrapulmonic air 

 increases in volume and becomes nearly saturated with water, but 

 if the necessary corrections are made for the temperature and pressure 

 and if the aqueous vapor is driven off, its volume is slightly less than 

 that of the inspired air (Hso part). This loss is accounted for by the 

 fact that a small portion of the oxygen is not given off as carbon dioxid, 

 but is either united with the sulphur of the proteins or is used in the 

 oxidation of the hydrogen. In the latter case it reappears as water. 

 It will be seen, therefore, that the body loses a certain amount of its 

 heat in the form of bound heat, because a portion of it is set aside for 

 the purpose of warming the air in the pulmonary passages, and a 

 portion for the purpose of converting the water into the gaseous state. 

 This aqueous vapor in the expiratory air is of considerable physiological 

 importance, because at 37° C. its tension amounts to 50 mm. Hg. 

 Assuming, therefore, that dry air is being breathed at the ordinary 

 pressure of 760 mm. Hg, the tension in the deeper recesses of the lungs 

 would amount to only 760 — 50 = 710 mm. Hg. Thus, the lungs 

 serve not only as a means of regulating the body-temperatm-e, but 

 also as a means of adjusting the water content of the tissues. The 

 expired air is also prone to contain extraneous material, consisting 

 chiefly of fragments of the lining of the pulmonary passage. 



The Interchange of the Gases Between the Tidal Air and the 

 Blood. — It has previously been shown that the quantity of air shifted 

 with each respiratory movement is relatively small, amounting on 

 an average to only 500 c.c. For this reason, it must be evident that 

 only the outer respiratory passage is ventilated with each respiration, 

 while the air in the infundibula remains stationary. Consequently, 

 the interchange of the gases between the outside air and the blood, 

 which is commonly designated as ''external" respiration, consists 

 in reality of two processes, namely (a) the shifting of the tidal air in 



