586 GENERAL METABOLISM [CH. XXXIX. 



to 6 '3 parts of proteid ; or 1 gramme of nitrogen to 30 grammes of 

 flesh. 



Fat and carbohydrate. Subtract the carbon in the metabolised 

 proteid (proteid contains 54 per cent, of carbon) from the total 

 carbon eliminated by lungs, skin, bowels, and kidneys, and the 

 difference represents fat and carbohydrate that have undergone 

 metabolism. 



The Discharge of Carbon. 



The influence of food on the rate of discharge of carbonic acid 

 is immediate. The increase after each meal, which may amount 

 to 20 per cent., reaches its maximum in about one or two hours. 

 This effect is most marked when the diet consists largely of carbo- 

 hydrates. 



About 95 per cent, of the carbon discharged leaves the organism 

 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 

 0*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 



"respiratory quotient" (by volume) =^ - 2 -- ex P 1 ^ . would be equal 



to 1. 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 78 grammes are 

 formed from the hydrogen of the fat); the respiratory quotient is 

 then 0'84. In hibernation the respiratory quotient sinks lower than 

 in any other known condition (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.) 



