CHANGES PRODUCED IN AIR ONCE BREATHED 373 



find how great its gaseous interchanges with the air are 

 during work and rest, waking and sleeping, while fasting 

 or digesting, and so on; but when it comes to be decided 

 what organs are concerned in each case in producing the 

 greater or less exchange, and how much of the whole is due 

 to each of them, the question is one far more difficult to 

 settle and still very far from completely answered. 



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 about 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 calculated as about equal to 50 calories 

 in twenty-four hours; a calorie (see Physics) 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 readily 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 tem- 

 perature and saturation of the inspired air: the cooler and 

 drier this is, the more water will it gain when breathed. 

 On an average 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 appear again 

 outside it when the water vapor condenses fiee Physics), 



