1250 PHYSIOLOGY 



The exact condition of the urate, however, will depend on the reaction 

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

 with the formation of uric acid in the following way : 



MHU + MH 2 P0 4 = H 2 U + M 2 HP0 4 . 



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

 secretion 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 phos- 

 phates, 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 sweetbreads, 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 forming the nitrogenous excreta of birds and 

 reptiles is formed. 



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

 hypoxanthine, 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 

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

 kidneys the benzoic acid is conjugated with glycine to form hippuric 

 acid. Thus 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 



HNCH 4 COOH 



