URINE. 267 



excreted under normal conditions, by an adult man in 24 hours is 

 about 30 grams ; women excrete a somewhat smaller amount. The 

 excretion is greatest in amount after a diet of meat, and least in 

 amount after a diet consisting of non-nitrogenous foods; this is 

 due to the fact that the last mentioned diet has a tendency to de- 

 crease the metabolism of the tissue proteins and thus cause the out- 

 put of urea under these conditions to fall below the output of urea 

 observed during starvation. The output of urea is also increased 

 after copious water- or beer-drinking. This increase is probably 

 due primarily to the washing out of the tissues of the urea previ- 

 ously formed, but which had not been removed in the normal proc- 

 esses, and, secondarily to a stimulation of protein catabolism. 



Urea may be formed in the organism from amino acids such as 

 leucine, glycocoll and aspartic acid : it may also be formed 

 from ammonium carbonate (NH 4 ) 2 CO 3 or ammonium carbamate, 

 H 4 N-OCONH 2 . 



There are differences of opinion regarding the transformation of 

 the substances just named into urea but there is rather conclusive 

 evidence that at least a part of the urea is formed in the liver ; it may 

 be formed in other organs or tissues as well. 



Urea crystallizes in long, colorless, four or six-sided, anhydrous, 

 rhombic prisms (Fig. 85, p. 266), which melt at 132 C. and are 

 soluble in water or alcohol and insoluble in ether or chloroform. 

 If a crystal of urea is heated in a test-tube, it melts and decomposes 

 with the liberation of ammonia. The residue contains cyanuric 



C-OH 



/\ 



N N 



C C- 



acid, and biuret, 



HOC C-OH 



N 



NH 



\ 



NH 



NH 2 



