142 



BIOLOGICAL CHEMISTRY 



of carbohydrate and fat oxidised. Knowing this proportion 

 and the total corrected amounts of carbon dioxide and of 

 oxygen the absolute amounts of carbohydrate and fat are 

 easily calculated. 



Each gram of nitrogen in the urine requires 8-471 grams or 

 5-923 litres of oxygen and gives rise to 9' 347 grams or 4754 

 litres of carbon dioxide, whilst the values for the oxidation of 

 carbohydrate and fat are given in the following table : * 



TABLE XXVII 



Showing relative amounts and heat value of glycogen and fat for 

 different respiratory quotients. 



Per i Litre of Oxygen. 



A respiratory quotient above one may be caused by the 

 conversion of carbohydrate into fat when the volume of 

 carbon dioxide given off is actually greater than the amount 

 of oxygen taken in. 



The energy balance-sheet is constructed on similar principles. 

 The energy value of the food can be calculated by combustion 

 of the same substances outside the body. The value for 

 carbohydrates (4*1 C.) and for fats (9-3 C.), is the value for 

 complete combustion, but for proteins a lower value is taken 

 because some of the carbon and hydrogen of the proteins is 

 excreted combined with the nitrogen. Each gram of protein 

 gives rise to one-third of a gram of urea. Therefore the heat 

 value for one gram of protein in the body (4-1 C.) is the heat 

 value for complete combustion of one gram of protein less 

 the heat value of one-third of a gram of urea. 



The heat loss is composed of a series of items. There is the 

 amount of heat required to raise the food and drink to body 

 temperature, the amount of heat required to warm the air 

 during respiration, the amount of heat required r to evaporate 

 water from the lungs and skin, the amount of 'heat lost by 

 radiation and convection, and the heat equivalent of the 

 external work performed. 



* A. Krogh, The Respiratory Exchange of Animals and Man. Long- 

 mans, Green & Co., 1916, p. 7. 



