344 ESSENTIALS OF PHYSIOLOGY. 



END PRODUCTS OF METABOLISM. 



The whole of the carbon in fat and carbohydrate, and a large pro- 

 portion of that in protein, is removed from the body in a completely 

 oxidised form as carbonic acid, while the nitrogen of protein leaves the 

 body in the form of urea, uric acid, and other substances, which are 

 excreted in the urine. Since protein contains about 1 6 per cent, nitrogen, 

 it is possible, by determining the total a'mount of nitrogen in the urine, 

 to calculate the amount of protein from which it has been derived ; 

 each gram of nitrogen in the urine represents the breaking down 

 in the body of 6*3 grams of protein. By measuring the output of 

 nitrogen in the urine and the amount of carbonic acid discharged from 

 the lungs daily, the total amount of protein, fat, and carbohydrate 

 which has been oxidised in the body can be calculated, if the respiratory 

 quotient is known. In health the amount of carbon and nitrogen thus 

 removed from the body is equivalent to the amount taken in with the 

 food, provided that the weight of the individual" remains steady. If 

 the individual is putting on weight, some of the carbon taken in the 

 food is retained in the body as fat or carbohydrate ; when weight is 

 being lost, more carbon, and possibly more nitrogen, will be discharged 

 than are taken in with the food. 



We may now consider the changes taking place in the various food- 

 stuffs after their absorption from the digestive tract. 



SECTION" II. 



THE METABOLISM OF FAT. 



The fats taken by the mouth, after being hydrolysed in the 

 digestive tract and re-synthesised in the walls of the villi, enter the 

 thoracic duct as neutral fats and pass into the blood stream. For a 

 short time after a fatty meal the fats circulate in the blood stream, and 

 may give the blood a slightly milky appearance, but they are rapidly taken 

 up and stored in the subcutaneous tissues and omentum, which serve 

 as depots in which the fat not immediately required by the body can 

 be kept. The composition of the subcutaneous fat thus reflects that 

 of the fat in the food ; and if abnormal and easily recognisable fats 

 are given by the mouth, they can be identified shortly afterwards in 

 the fat in the subcutaneous tissue. For this purpose, erucic acid or 

 fats containing iodine may be used. In man the greater part of the 

 fat deposited in the body is derived from the fat in food, but it can be 

 formed, and in herbivora is mainly formed, from carbohydrates. This 



