SEC. 2. CHANGES OF THE AIR IN RESPIRATION. 



During its stay in the lungs, or rather during its stay in the 

 bronchial passages, the tidal air (by means of diffusion chiefly) 

 effects exchanges with the stationary air ; in consequence the ex- 

 pired air differs from inspired air in several important particulars. 



1. The temperature of expired air is variable, but under 

 ordinary circumstances is higher than that of the inspired air. At 

 an average temperature of the atmosphere, for instance at about 

 20 C., the temperature of expired air is, in the mouth 33*9, in the 

 nose 35*3. When the external temperature is low, that of the ex- 

 pired air sinks somewhat, but not to any great extent, thus at 

 - 6*3 C. it is 29'8 C. When the external temperature is high, the 

 expired air may become cooler than the inspired, thus at 41 '9 it 

 was found by Valentin to be 38*1. The exact temperature in fact 

 depends on the relative temperatures of the blood and inspired air, 

 and on the depth and rate of breathing. 



2. The expired air is loaded with aqueous vapour. The point 

 of saturation of any gas, that is, the utmost quantity of water 

 which any given volume of gas can take up as aqueous vapour, 

 varies with the temperature, being higher with the higher 

 temperature. For its own temperature expired air is according 

 to most observers saturated with aqueous vapour. 



3. When the total quantity of tidal air given out at any expira- 

 tion is compared with that taken in at the corresponding inspi- 



