412 THE HUMAN BODY 



Ordinary atmospheric air contains in addition 4 volumes of 

 carbon dioxid in 10,000, or 0.04 in 100, a quantity which, for prac- 

 tical purposes, may be neglected. When breathed once, such 

 air gains rather more than 4 volumes in 100 of carbon dioxid, 

 and loses a little less than 5 of oxygen. More accurately, 100 vol- 

 umes of expired air after drying contain: 



Oxygen 16. 



Nitrogen 79. 



Carbon dioxid 4.4 



Since 10,800 liters (375 cubic feet) of air are breathed in twenty- 

 four hours and lose 5 per cent of oxygen, the total quantity of 

 this gas taken up in the lungs daily is 10,800 X 5 -f- IPO = 540 

 liters. One liter of oxygen measured at C. (32 F.) and under a 

 pressure equal to one atmosphere, weighs 1.43 grams, so the total 

 weight of oxygen taken up by the lungs daily is 540 X 1.43 = 772 

 grams (27 ounces). 



The amount of carbon dioxid excreted from the lungs being 

 4.4 per cent of the volume of the air breathed daily, is 10,800 x 

 4.4 -f- 100 = 475 liters measured at the normal temperature and 

 pressure. This volume weighs 930 grams, or 32.5 ounces. If all 

 the oxygen taken in were breathed out again as carbon dioxid 

 the volume of the latter should equal that of the oxygen breathed 

 in. The discrepancy results from the fact that not all the oxygen 

 combines with carbon; some of it unites with hydrogen to form 

 water. The water thus formed simply adds itself to the general 

 water content of the Body, and has no bearing on the amount dis- 

 charged in the expired air; this latter depending, as already stated, 

 on the rate of evaporation from the lung surface. 



If the expired air be measured as it leaves the Body its bulk 

 will be found greater than that of the inspired air, since it not 

 only has water vapor added to it, but is expanded in consequence 

 of its higher temperature. If, however, it be dried and reduced 

 to the same temperature as the inspired air its volume will be 

 found diminished, since it has lost 5 volumes per cent of oxygen 

 and gained only 4.4 of carbon dioxid. In round numbers, 100 

 volumes of dry inspired air at zero, give 99 volumes of dry expired 

 air measured at the same temperature and pressure. 



Ventilation. Since at every breath some oxygen is taken from 



