COMPOSITION AND CHARACTERS OF URINE 1119 



reaction of the urine. A bi-urate, with acid sodium phosphate, is decom- 

 posed with the formation of uric acid in the following way : 



MHU + MH 2 P0 4 = H 2 U + M 2 HPO 4 . 



Thus the quadri-urates present in the urine immediately after its secre- 

 tion will tend to undergo spontaneous decomposition into uric acid and the 

 bi-urate, and the latter itself may be decomposed with the formation of 

 uric acid and alkaline phosphate. We see therefore that when the urine is 

 acid, i.e. there is a predominance of acid phosphates, there will be a tendency 

 to the precipitation of uric acid in the urinary passages. If, however, the di- 

 sodium phosphate be in excess the uric acid may be kept in solution as the 

 quadri-urate or even as the bi-urate. 



The uric acid of the urine is derived almost entirely from the purine 

 metabolism of the body. The uric acid may be endogenous or exogenous, 

 i.e. may be derived from the breaking down of the nucleins of the cells or 

 by a direct transformation of the nucleins contained in the food. The 

 amount passed daily varies between 0-4 and 1 grm., according to the nature 

 of the diet. It is not absent from the urine even during complete starvation. 

 It is increased when foods are ingested rich in nucleins, such as liver or sweet- 

 breads, or in any other precursors of uric acid, e.g. hypoxanthine, such as 

 meat or meat extract. We have no evidence that the urinary uric acid 

 in the mammal is formed by synthesis, though this is the manner in 

 which the greater part of the uric acid excreted by birds and reptiles is 

 formed. 



Small traces of purine bases also occur in urine, namely, xanthine. hypo- 

 xanthine, and adenine. When tea and coffee are taken the methyl-purines 

 may occur, namely, caffeine, theobromine, and their derivatives. 



HIPPURIC ACID is a frequent, though not a constant, constituent of 

 human urine. It is derived from benzoic acid or from an aromatic sub- 

 stance which on oxidation can give rise to benzoic acid. In the kidneys 

 the benzoic acid is conjugated with glycine to form hippuric acid. The 

 amount of hippuric acid excreted in the day may vary between 0-1 and 

 1 grm. After a diet rich in fruit or vegetables its amount may rise to 2 grm. 

 It is present in considerable quantities in the urine of herbivora and may be 

 most easily prepared from horses' urine. Hippuric acid has the formula : 



C 6 H 5 CO 



HN 



It can be obtained in milk-white crystals (Fig. 520), which are only slightly 

 soluble in cold water, but easily soluble in alcohol, ether, and acetic acid. 

 It is insoluble in petroleum, ether, and benzol. On heating, it is broken up 

 into benzoic acid and glycine. On heating with concentrated nitric acid 

 it forms nitro-benzol, which can be recognised by its characteristic smell of 

 bitter almonds. 



In order to extract it from the urine, the urine is made alkaline with sodium carbo- 

 nate, filtered, and the filtrate evaporated to a syrupy consistence. This is then treated 



