SECRETION. 



443 



Among the "substances dissolved" in urine we find: urea, uric, 

 hippuric, lactic, and oxalic acids, and ammonia; also creatin, chlo- 

 rides, sulphates, phosphates, with the bases potassium, sodium, cal- 

 cium, and magnesium. 



Urea (CO[NH 2 ] 2 ) is the diamide of C0 2 ; that is, a carbamide. 



Urea greatly prevails over the other constituents of the urine, 

 since in normal urine it forms nearly one-half of the solids. Nearly 

 one-half of urea is nitrogen. It is the principal representative of 

 the waste of the nitrogenous tissues. 



Urea is inodorous, fresh, bitter, neutral, very soluble in water 

 and alcohol, but almost insoluble in ether. It crystallizes quickly 





Fig. 167. Urea from Human Urine. (FuNKE.) (From Tiger- 

 stedt's "Human Physiology," copyright, 1906, by D. Appleton and Com- 

 pany.) 



into needles; slowly, into quadrangular prisms of the rhombic sys- 

 tem. Urea fuses and decomposes at 248 F., with the development 

 of ammonia. 



Urea is very rich in nitrogen. The nitrogen that finds its way 

 from the body through the urine as a vehicle amounts to about 15 

 grams in twenty-four hours. This represents practically all of the 

 nitrogenous waste of the economy, since less than 1 gram finds egress 

 from all other channels taken collectively. The total amount of 

 nitrogen is estimated by the Kjeldahl process. 



Among the combinations with acids and bases of which urea 

 is capable, those with nitric and oxalic acids are important. It is 

 precisely these which are most commonly employed in the extraction 

 of urea. With nitric acid, nitrate of, urea is formed, which crystal- 

 lizes in lozenge-shaped crystals. With oxalic acid, urea forms urea 



