CHANGES IN RESPIEED AIR. 251 



see what it is that happens to this vast quantity of air 

 breathed daily by each one of us; what we have taken out 

 of it, and what we have given off to it. 



The Changes produced in Air by being once breathed. 

 These are fourfold changes in its temperature, in its mois- 

 ture, in its chemical composition, and in its volume. 



Temperature Changes. The air taken into the lungs is 

 nearly always cooler than that expired, which has a tem- 

 perature of about 36 C. (97 F.). The temperature of a 

 room is usually about 21 C. (70 F.). The warmer the 

 inspired air, the less the heat which is lost to the body in 

 the breathing process. 



Changes in Moisture. Inspired air always contains more 

 or less water vapor, but is rarely saturated that is, rarely 

 contains so much but it can take up more without showing 

 it as mist; the warmer air is, the more water vapor it re- 

 quires to saturate it. The expired air is nearly saturated 

 for the temperature at which it leaves the body, as is read- 

 ily shown by the vapor deposited when it is slightly cooled, 

 as when a mirror is breathed upon; or by the clouds seen 

 issuing from the nostrils on a frosty day, these being due 

 to the fact that the air as soon as it is cooled cannot hold 

 all the water vapor which it took up when warmed in the 

 body. We therefore conclude that air when breathed gains 

 water vapor and carries it off from the lungs. The quan- 

 tity of water thus removed from the body is about nine 

 ounces each twenty-four hours. 



Chemical Changes. The most important changes 

 brought about in the breathed air are those in its chemical 



What changes are produced in the air on its being once breathed ? 

 How does expired air differ in temperature from inspired? 

 How does expired air differ from inspired in moisture? How 

 much water is evaporated from the lungs daily? 



