39 PHYSIOLOGICAL CHEMISTRY 



the substance. Urea is also found in nearly all the fluids and in many 

 of the tissues and organs of mammals. The amount excreted, under 

 normal conditions, by an adult man in 24 hours is about 30-35 grams. 

 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 urea output is regulated by the protein ingestion. It is 

 true also that a non-nitrogenous diet has a tendency to decrease the 

 metabolism of the tissue proteins and thus cause the output of urea under 

 these conditions to fall below the output of urea observed during starva- 

 tion. The output of urea is also increased after copious water- or beer- 

 drinking. The increase is probably due primarily to the washing out of 

 the tissues of the urea previously formed, but which had not been re- 

 moved in the normal processes, and secondarily to a stimulation of 

 protein catabolism. 



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

 cine, glycocoll, and aspartic acid: it may also be formed from ammonium 

 carbonate (NH^COs or ammonium carbamate, H 4 N.O.CO.NH 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. 123), which melt at i32C. 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 acid, 



C.OH 



N N 



II I 

 HO.C C.OH 



V 



N 



NH 2 



I 

 and biuret, C = 



NH 



NH 



