21 



Phosphates. The phosphates consist of alkaline and 

 earthy salts in the proportion of 2 to 1. The latter are insol- 

 uble in an alkaline medium and are precipitated when acid 

 urine becomes alkaline. They are insoluble in water, but 

 soluble in acids ; in acid urine they are held in solution by free 

 CO2. The alkaline phosphates (sodium and potassium) are 

 very soluble in water, and they never foriii urinary deposits. 

 The earthy are phosphates of calcium (Cag P0^)o (abundant) 

 and magnesium (MgHPO^ plus THaO) (scanty). An alkaline 

 medium precipitates them although not in the form in which 

 they occur in the urine. 



The excretion of phosphates in the urine is largely de- 

 pendent upon the amount of calcium ingested; the more cal- 

 cium the food contains, the less phosphoric acid appears in the 

 urine, and the more in the feces. This is due on the one hand 

 to the tendency on the part of calcium to form insoluble cal- 

 cium phosphates in the intestinal tract and in this way to 

 prevent the absorption of the food phosphates ; on the other 

 hand, to the well-established tendency 6f calcium salts to be 

 excreted into the bowel and not into the bladder; one must 

 imagine in the latter case that calcium salts circulating in the 

 blood combine with circulating phosphoric acid and bear the 

 latter with them into the bowel. 



The fact is of some therapeutic importance in the treat- 

 ment of nephrolithiasis due to uric acid calculi, for the admin- 

 istration of calcium salts in this affection by bearing much 

 phosphoric acid into the bowel, leads to the excretion of less 

 phosphoric acid in the urine, and hence of normal and basic 

 instead of acid phosphates; and as the latter precipitate and 

 the former dissolve uric acid, it will be spen that by giving 

 calcium we prevent the precipitatipn of crystalline uric acid 

 and urate deposits in the urinary passages. (Croftan). 



"Where considerable calcium is present in the food the 

 excretion of phosphates in the urine is minimum, especially if 

 the urine is alkaline because of the presence of sodium or 

 potassium salts. In herbivorous animals where the urine is 

 alkaline and where considerable quantities of phosphorus con- 

 taining food are eaten, very little phosphate is excreted in the 

 urine. 



