348 MANUAL OF PHYSIOLOGY. 



considerably; according to the season of the year, etc., but almost 

 always in this country it is lower than that of our bodies. 



EXPIRED AIR. 



The following are the notable characters in the tidal air on its 

 leaving the air-passages: 



1. It is rich in CO 2 , containing on an average 4.38 per cent, in 

 quiet breathing. 



2. It is poor in O, containing about 4.5 per cent, less than the 

 atmosphere. 



3. A slight increase in the N has been observed, possibly, the 

 outcome of nitrogenous metabolism. 



4. The temperature of the air is approximated to that of the 

 body, and it therefore commonly exceeds the temperature of the 

 air inspired. The air on leaving the air-passages is about 36-.5 

 C. This is not much influenced by the temperature of the atmo- 

 sphere, as may be seen from Valentine's Table : 



Temperatures of Atmosphere. and of Expired Air. 

 6.3 C. +29.8C. 



+ 17.0C. +36.2C. 



+ 44.0C. +38.5C. 



It can be seen from the last statement that very hot air (-f 44 

 C.) if breathed is cooled in its transit through the air-passages. 



5. In quiet breathing the expired air is saturated with moisture; 

 in rapid breathing this is not the case. It must be remembered 

 that the air when warm is capable of holding a greater quantity 

 of vapor than when it was inspired. The difference can be best 

 appreciated in cold weather, when the vapor of the warm expired 

 air is condensed on meeting the cold atmosphere. Great quanti- 

 ties of water and heat are given off in producing this saturation. 



6. If the tidal air be dried and cooled and measured at a cer- 

 tain pressure before and after respiration, it is found that the ex- 

 pired air has lost about -^ of its volume. But owing to the ex- 

 pansion from the increased temperature and the presence of the 

 vapor, the volume of air expired is greater than that inspired. 



If the oxygen were all used to make CO 2 , these volumes ought 



