286 URINE. [CH. XII. 



E. Urea. 



Urea is the compound in which the greater part of 

 the nitrogen is normally excreted in man. The percentage 

 of the urinary nitrogen in the form of urea varies. Normally 

 it is about 86 per cent., but in starvation, or on a diet 

 deficient in proteins, it is only about 60 per cent. It is 

 also low in cases of diabetes accompanied by acidosis 

 (owing to the relatively high percentage of ammonia), and 

 also in certain cases of hepatic disorder, notably acute 

 yellow atrophy of the liver, owing to the non-formation 

 of urea by the disordered liver, its seat of formation in the 

 body. 



The total amount excreted per diem by a normal man 

 on an average diet containing 100 grams, of protein is 

 30 grams. 



Urea is also known as carbamide, since it is the diamide 

 of carbonic acid. 



o-c - OH o r NH * 



-OH -NH^ 



Carbonic acid. Urea. 



Urea crystallises in water-free, colourless, long needles, 

 or in four-sided prisms of the rhombic system, which melt 

 and decompose at 130 132 C. 



It is soluble in all proportions in hot water, and to 

 the extent i : i in cold water. In cold alcohol it is soluble 

 to the extent of i : 5. It is also soluble in acetone. In- 

 soluble in pure ether and chloroform. The solutions are 

 neutral in reaction. 



It forms crystalline compounds with acids. The two 

 most important are urea nitrate CH 4 N 2 O.HNO 3 , insoluble 

 in strong nitric acid, and urea oxalate (CH 4 N 2 O) 2 , C 2 H 2 O 4 , 

 insoluble in oxalic acid. 



It forms compounds with the salts of the heavy metals, 

 especially with mercuric nitrate (see below, Ex. 341). 



With reducing sugars relatively stable compounds 

 are formed, called ureides. They are of importance in 

 connection with the estimation of urea in diabetic urine. 



