584 GENERAL METABOLISM. [CH. xxxix. 



as carbonic acid. The total insensible loss (= carbonic acid + 

 water given off -oxygen absorbed) amounts in man to about 25 

 grammes per hour. Of the total hourly discharge of carbonic 

 acid, less than o'5 per cent, is cutaneous. The hourly discharge 

 of carbonic acid in a man at rest is about 32 grammes, the 

 weight of oxygen absorbed being 25 to 28 grammes in the same 

 time. The hourly discharge of watery vapour is about 20 

 grammes. 



As a volume of carbonic acid (C0 2 ) contains the same weight of 

 oxygen as an equal volume of oxygen (0 2 ), it is obvious that, if 

 all the absorbed oxygen were discharged as carbonic acid, the 



C0 2 expired 

 "respiratory quotient" (by volume) =^. r r~ 5 would be equal 



to i. This, however, is not the case, the volume of oxygen 

 absorbed being in excess of the carbonic acid discharged. In 

 animals fed exclusively on carbohydrates (this would only be 

 possible for a short time) equality is approached. The excess of 

 oxygen is greatest when the diet consists largely of fats. 



On a mixed diet, comprising 100 grammes of proteid, 100 of 

 fat, and 250 of carbohydrates, with a carbonic acid discharge of 

 770 grammes daily, and a daily assumption of 666 grammes of 

 oxygen, 560 grammes of the oxygen are discharged in the carbonic 

 acid, about 9 in urea, and 97 grammes in the form of water (of 

 which 7 8 grammes are formed from the hydrogen of the fat) ; 

 the respiratory qiiotient is then o'84. In hibernation the 

 respiratory quotient sinks lower than in any .other known con- 

 dition (often less than 0*5), for the animal then lives almost 

 entirely on its own fat. The discharge of carbonic acid is 

 increased by muscular work, and the respiratory quotient also 

 rises. Diminution of the surrounding temperature causes increased 

 discharge of carbonic acid. (These points are all discussed more 

 fully in Chapter XXIV.) 



The Discharge of Nitrogen. 



In man the minimum daily allowance of nitrogen is 15 

 grammes, or 0-02 per cent, of the body-weight ; in the carnivora 

 about o'i per cent. ; in the ox, as an instance of a herbivorous 

 animal, 0*005 P er cen ^- I n certain races of mankind (e.g. coolies) 

 the nitrogen requirement is less than in Europeans. The reason 

 why this is so is not understood. 



Some recent experiments by Hirschfeld have shown that for a 

 short time nitrogenous equilibrium can be maintained on a 

 smaller daily supply of nitrogen than 15 grammes. But 



