THE RESPIRATORY EXCHANGE 151 



In the case of proteins, owing to their varying composi- 

 tion, the R.Q. is not constant. The average figure is 0-8. 

 The proportion of nitrogen in protein is sufficiently con- 

 stant to allow of the nitrogen excreted being a measure 

 of the protein catabohsed, one gm. of nitrogen corre- 

 sponding to 6"2 guis. of protein which yields on oxidation 

 5-9 htres of oxygen, and 4-8 litres of COg. 



Knowing then the total respiratory exchange, and deduct- 

 ing from tliis the exchange which is due to the catabohsm 

 of protein as estimated from the urine, we are left with the 

 respiratory exchange which represents the combustion of 

 non-protein material. It only remains to determine how 

 much is due to carbohydrates and how much to fats. 

 This can be estimated from the R.Q. obtained from the 

 non-protein respiratory exchange. If the figure obtained 

 is 1-0, carbohydrates only are being metabolised; if 0-7, 

 fats only, any intervening figure representing a certain 

 proportion of carbohydrates and fats. 



The following example (from Krogh) will make this clear. 



Total gaseous exchange = 405 litres O2 and 331 litres CO2 



N. excreted, 34-93 gms. cor- 

 responding to 206-9 ,, „ „ 166 ,, ^ 



Non-protein gaseous ex- 

 change 198-1 ,, ,, „ 165 „ „ 



165 

 Non-protein R.Q. = ' = 0-833. 



The figure 0-833 corresponds to a combustion of — 

 0-51 gms. carbohydrate 1 ,•<- t 



and 0-293 „ fat / P^^ ^^'^^ °^ «^^^S'^- 



The subject is therefore catabolising — 



Protein . . , 34-93 X. 6-2 = 218 gms. 



Carbohydrate. . 0-51 x 198-1 -= 101 '., 



Fat . . . 0-293 x 198-1 =. 5S „ 



Now, as stated above, 1 gm. protein on combustion gives off 

 50 C, 1 gm. carbohydrate 4-1 C, and 1 gm. fat 9-3 C. 

 The total heat- production in this case is therefore — 



(218 x 5-0) + (101 X 4-1) + (58 X 9-3) 

 -= 2043-5 Calories. 



