URINE 559 



are daily eliminated, while iu the ox as much as 20 litres may 

 be passed. 



1. Nitrogenous Constituents. 



The waste nitrogen of the body occurs in the urine in 

 different substances, the origin of each of which has been 

 considered. 



A. Urea. — Urea is the most abundant constituent of the 

 urine. Its chemistry and mode of formation have been discussed 

 on p. 359. The amovint excreted depends wpon the amount of 

 2?rotein taken in the food, and for this reason, in man during 

 fasting, the excretion may fall as low as 10 grms. per diem, while 

 on a diet containing an average amount of proteins, about 86 grms. 

 of urea — 16'7 grms, of nitrogen — are excreted. On a normal 

 diet about 90 per cent, in the dog and 80 per cent, in the horse 

 of the waste nitrogen is excreted as urea, but, when the protein 

 intake is decreased and non-nitrogenous food is substituted, 

 the proportion of urea-nitrogen may fall as low as 60 per cent, 

 of the whole (p. 562). 



When urine is allowed to stand, micro-organisms are apt to 

 gain access, and to cause a hydration of the urea, whereby it is 

 changed into ammonium carbonate — 







11 II 



H,N— C— NH,-F2H,0 = H,N— O— C— 0— NH, 



The urine is thus made alkaline, and the earthy phos- 

 phates are precipitated. The magnesium phosphate combines 

 with the ammonia to form ammonium-magnesium-phosphate, 

 XH^MgPO^ -I-6H2O (triple phosphate), which crystallises in 

 characteristic prism-like crystals (fig. 225). 



B. Non-Urea Nitrogen. — Some 20 per cent, of nitrogen which, 

 on an ordinary diet, is not excreted as urea is distributed in — 



1. Ammonium Salts. — In herbivora a very small proportion 

 of nitrogen is normally excreted as ammonium salts. But, 

 under certain conditions, the proportion is increased (p. 554). 

 Anything which tends to raise the Ch of the blood, e.g. the 

 formation of /3-oxybutyric acid (p. 358), causes an in- 

 creased excretion of ammonia — the ammonia being formed 



