RESPIRATION: THE GASEOUS INTERCHANGES 371 



The Changes Produced in Air by Being Once Breathed. These 

 are fourfold changes in its temperature, in its moisture, in its 

 chemical composition, and its volume. 



The air taken into the lungs is nearly always cooler than that 

 expired, which has a temperature of about 36 C. (97 F.). The 

 temperature of a room is usually less than 21 C. (70 F.). The 

 warmer the inspired air the less, of course, the heat which is lost 

 to the Body in the breathing process; its average amount is calcu- 

 lated as about equal to 50 Calories in twenty-four hours ; a Calory 

 being as much heat as will raise the temperature of one kilogram 

 (2.2 Ibs.) of water one degree centigrade (1.8 F.). 



The 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 requires to saturate it. The expired air is 

 nearly saturated for the temperature at which it leaves the Body, 

 as is already shown by the water 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. Air, therefore, 

 when breathed once, gains water vapor and carries it off from the 

 lungs; the actual amount being subject to variation with the 

 temperature and saturation of the inspired air : the cooler and drier 

 that is, the more water will it gain when breathed. On an aver- 

 age the amount thus carried off in twenty-four hours is about 255 

 grams (9 ounces) . To evaporate this water in the lungs an amount 

 of heat is required, which disappears for this purpose in the Body, 

 to reappear again outside it when the water vapor condenses. 

 The amount of heat taken off in this way during the day is about 

 148 Calories. The total daily loss of heat from the Body through 

 the lungs is therefore 198 Calories, 50 in warming the inspired air 

 and 148 in the evaporation of water. 



The most important changes brought about in the breathed 

 air are those in its chemical composition. Pure air when com- 

 pletely dried consists in each 100 parts of: 



By Volume. By Weight. 



Oxygen 21 23 



Nitrogen 79 77 



