THE URINARY SYSTEM. 385 



amount varying, as seen in the Table on p. 357, with the quantity of 

 protein food consumed ; in starvation urea is derived entirely from 

 the breaking down of the tissue proteins. 



AMMONIA. 



The ammonia normally found in the urine represents the small 

 amount which escapes conversion into urea by the liver, and is but 

 little affected by the amount of protein in the diet. In acidosis the 

 abnormal acids formed in the body combine to a large extent with 

 ammonia in the blood, being excreted as ammonium salts in the urine, 

 and the amount of urea is correspondingly diminished. The amount 

 of nitrogen excreted as ammonia in these circumstances is sometimes 

 20 per cent, or more of the total urinary nitrogen. 



CREATININE AND CREATINE. 



Creatinine is an anhydride of creatine, which occurs in muscle, and 

 it may be formed from the latter by boiling it with strong hydrochloric 



NH- -, 



acid. It has the formula and gives a red 



NH = C-N(CH 3 ).CH 2 .CO, 



colour with caustic potash and picric acid (Jaffe's test). When creatine 

 is taken by the mouth, either as a pure substance or in meat, some of 

 it may appear in the urine as creatinine. With this exception, the 

 creatinine in urine is derived solely from the endogenous metabolism 

 of protein ; and the amount excreted in the urine by the same person 

 from day to day is remarkably constant, and serves as an accurate 

 index of the extent of endogenous protein metabolism. 



The origin of creatinine and its relation to the creatine present in 

 muscle are not fully understood, though creatinine appears in the 

 urine in increased amount in fever, and in other conditions in which 

 rapid wasting of muscular tissue is taking place. Creatine also, though 

 not usually present in the urine of adults, is found during starva- 

 tion, in diabetes, in acute fevers, and in women during involution of 

 the uterus. It is possible that normally the creatinine in the urine is 

 formed from creatine, and that in the conditions just mentioned an 

 increased amount of creatine is set free by protein decomposition, some 

 of which is converted into creatinine, while a portion is excreted in an 

 unaltered form. 



URIC ACID. 



Uric acid exists in urine in the form of biurates. On adding strong 

 hydrochloric acid to urine and allowing it to stand for twenty-four 

 hours, uric acid separates out as small pigmented s crystals having a 



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