METABOLISM. 



357 



ENDOGENOUS AND EXOGENOUS METABOLISM. 



When the composition of the urine excreted in 24 hours by a man 

 taking very little protein food is compared with that of the same 

 person when taking an abundance of protein food, -very great differences 

 are observed, and are shown in the following table : 



The amount of urea and sulphates in the urine is greatly 

 increased by a protein meal, whereas the creatinine and ammonia are 

 but little affected, this difference being due to the fact that the 

 metabolism of protein in the body is of two kinds, endogenous and 

 exogenous. The metabolic changes concerned in the production of 

 creatinine and most of the uric acid are known as endogenous 

 metabolism, since these substances are formed by the breaking down 

 of protein in the tissues as part of their ordinary wear and tear, 

 and are unaffected by the amount of protein food eaten, unless this 

 contains creatinine or uric acid. On the contrary, the amount of urea 

 and inorganic sulphates in the urine depends chiefly upon the quantity 

 of protein in the food, the urea being formed from the ammonia set 

 free by the deamination of the amino-acids absorbed into the blood 

 stream during digestion. It thus represents a change taking place in 

 the amino-acids before they reach the muscles and other tissues, and is 

 quite independent of the metabolic changes in the tissues themselves. 

 For this reason, the formation of urea and sulphates is described as 

 exogenous metabolism. 



The correctness of this view is proved by the rapidity with which 

 urea is excreted in the urine after a protein meal; the excretion of 

 urea begins to increase within two hours after the meal, and within 

 five hours half the total nitrogen taken in with the food may be 

 excreted as urea. It would be almost impossible for the body to 

 have built up the amino-acids into the living tissues, and to have 



