ESSENTIALS OF PHYSIOLOGY. 



UREA. 



The presence of urea in urine may be shown by evaporating the 

 urine to dryness on a water bath, and extracting the residue with 

 acetone (which dissolves urea). On evaporation of the acetone extract 

 the urea crystallises out. 



Urea (CON 2 H 4 ) is a solid, crystallising in colourless rhombic prisms 

 which are easily soluble in water, alcohol, and acetone. When heated, 

 the crystals decompose, giving off ammonia and yielding a body called 

 biuret. Urea combines with nitric or oxalic acid, forming characteristic 

 crystals, and is decomposed by nitrous acid with the evolution of car- 

 bonic acid and nitrogen, or by alkaline sodium hypobromite, according 

 to the following equation : 



CON 2 H 4 + 3NaBrO = C0 2 + N 2 + 2H 2 + SNaBr. 



The carbonic acid is absorbed by the alkali, and the nitrogen is 

 given off ; and by collecting and measuring in a graduated cylinder the 

 amount of nitrogen evolved from 5 c.c. of urine, the percentage of urea 

 can be ascertained. Theoretically, 1 gram of urea yields 371 c.c. 

 of nitrogen at C. and 760 mm. Hg, but actually only 354 c.c. are 

 evolved from 1 gram of urea in urine. 



Another and more accurate method of estimating the amount of urea is 

 to treat a known volume of urine with an extract of Soya bean, which converts 

 urea into ammonia ; the ammonia formed is passed into a known volume of 



acid, and the amount of uncombined acid is subsequently estimated. 



When urine is exposed to the air the urea soon becomes decom- 

 posed by micro-organisms, being converted into ammonium carbonate, 

 and the urine becomes strongly alkaline. 



The nitrogen present as urea usually forms about 85 per cent, of 

 the total nitrogen existing in one combination or other in the urine. 

 The total nitrogen in urine is estimated by KjeldahVs method as 

 follows : 



A known volume of urine is boiled with pure sulphuric acid until 

 all its carbon is fully oxidised, the nitrogen being converted into 

 ammonia, which combines with the acid. The solution is then made 

 alkaline with caustic potash and boiled, the ammonia, which distils off, 



N 

 being collected in a known volume of sulphuric acid. The amount 



of acid neutralised by the ammonia is determined by subsequent 



N 

 titration of the uncombined acid with caustic potash. 



Urea is the most abundant nitrogenous constituent of urine, its 



