CH. XII. 1 PHOSPHATES. 283 



ammonium ; on the other, as salts of the alkaline earths, 

 calcium and magnesium. About 3-9 grams, of P 2 O 5 are 

 excreted per diem in the urine. Phosphoric acid forms 

 three series of salts. The formulae for that of sodium and 

 calcium are 



Normal phosphate, Na 3 PO 4 : Ca^POJg. 



Mono-hydrogen phosphate, Na 2 HPO 4 : CaH(PO 4 ). 



Di-hydrogen phosphate, NaH 2 PO 4 : CaH 4 (PO 4 ) 2 . 



The three sodium salts and CaH 4 (PO 4 ) 2 are soluble in 

 water : the other two calcium salts are insoluble. The 

 normal and mono-hydrogen phosphates are alkaline in 

 reaction to litmus : the di-hydrogen phosphates are acid. 



The phosphates of the urine are derived partly from 

 the inorganic phosphates of the food, partly from the 

 oxidation of phosphorus-containing substances of the 

 food and tissues, such as nucleo-proteins, lecithins and 

 phospho-proteins, and partly also from the phosphates of 

 bone. The exact share played by these various compounds 

 in forming the urinary phosphates is difficult to deter- 

 mine owing to the fact that a proportion of the phosphates, 

 varying between 12 and 50 per cent., are excreted by the 

 bowel. In this connection it may be noted that alkaline 

 phosphates of the food are more likely to be excreted in 

 the urine than are earthy phosphates. 



The excretion of varying amounts of phosphates by 

 the kidney is one of the methods by means of which the 

 reaction of the body fluids is maintained in equilibrium. 

 An increased excretion is always seen in cases of acid 

 poisoning and in the acidosis associated with diabetes. 



As soon as the urine shews a certain grade of alka- 

 linity, precipitation of earthy phosphates takes place. 

 This is sometimes known as phosphaturia, but it is not 

 necessarily associated with an increase of phosphates in 

 the urine. In the phosphaturia of juveniles it is probable 

 that there is an excessive amount of calcium in the urine, 

 due to a defective excretion of the large intestine. 



