160 HUMAN PHYSIOLOGY 



hydrogen, hence all the inhaled oxygen is used in the oxidation 

 of the carbon. One litre of oxygen used furnishes i litre of 

 carbon dioxide. Proteids and fats do not contain sufficient oxygen 

 to oxidize all their hydrogen. If i litre of oxygen is used to 

 oxidize proteids, 800 cc of carbon dioxide are formed ; if used to 

 oxidize fats, only 700 cc are formed. 



The respiratory quotient can also be greater than one 

 when the amount of carbon dioxide is greater than that of 

 the oxygen taken up. This is the case when, in the body, 

 substances rich in oxygen, e.g. carbohydrates, are reduced 

 to products containing less oxygen, e.g. fats. 



The respiratory quotient can also be less than it is during 

 oxidation of pure fat. This occurs when the oxygen intro- 

 duced is stored up in the form of compounds rich in oxygen. 



The respiratory quotient is therefore subject to consider- 

 able variations. It is greatest during a carbohydrate diet, 

 smallest during a fat diet. But independently of the diet, 

 the respiratory quotient has been observed to undergo 

 periodic variations, for sometimes relatively more carbon 

 dioxide is given off; at another time relatively more oxygen 

 is taken up. This shows that the oxygen taken up is not 

 immediately used for the formation of carbon dioxide, but 

 first forms compounds rich in oxygen which are, at a later 

 time, completely oxidized to carbon dioxide and water. 



Determining the amount of the sulphur and phosphorus 

 in the income and outgo is also of value in studying the 

 proteid metabolism. 



The balance of water shows not only how much water has 

 been taken up and given off by the body, but also how much 

 water has been formed by combustion processes in the body. 



In regard to the substances excreted in the faeces, it must 

 still be mentioned that the faeces contain not only end- 

 products of metabolism, but also undigested and unabsorbed 

 parts of the food. The quantity of the latter substances 

 must be subtracted from the food taken up, for the foodstuffs 

 not absorbed cannot be included in the metabolism. But we 

 have not yet been able to separate the end-products of meta- 

 bolism present in the faeces from the merely unabsorbed 



