1166 PHYSIOLOGY 



micrococcus ureae, which is responsible for the ammoniacal change which 

 occurs in urine when exposed to the air. 



Urea, being the chief nitrogenous constituent of the urine, is the most 

 important index to the protein metabolism of the body. As we have seen, 

 urea may be regarded as partly exogenous, partly endogenous. The greater 

 part of the 30 grm. excreted by a normal individual in the course of the day is 

 derived directly from the proteins of the foods, as a result of the deamination 

 of the amino-acids, which occurs shortly after their absorption. The 

 ammonia thus formed is combined with carbonic acid and carried to the 

 liver, where the ammonium carbonate undergoes dehydration with the pro- 

 duction of urea. Its amount will therefore be directly proportional to the 

 amount of protein taken as food. A smaller proportion is derived from 

 the breakdown of the tissues of the body. This endogenous moiety may 

 undergo considerable increase under any conditions which cause a rapid dis- 

 integration of the tissues. Thus in febrile conditions there is a large rise in 

 the urea output, even in a starving individual. 



In order to prepare urea from urine, advantage may be taken of the insolubility of 

 its combination with nitric acid. Urine is concentrated to about one-sixth of its bulk. 

 It is then cooled and treated with twice its volume of pure concentrated nitric acid, 

 care being taken to keep the whole mixture cool. Urea nitrate is precipitated. The 

 precipitate is collected, dried roughly by pressing between filter paper, and then rubbed 

 up with fresh barium carbonate. Barium nitrate is formed and urea set free. The 

 whole mass is dried and treated with hot absolute alcohol, in which the barium nitrate 

 is insoluble. On filtering off the barium nitrate a pure solution of urea is obtained 

 from which the urea will crystallise on allowing the alcohol to evaporate. 



CREATININE. Creatinine is a normal constituent of urine, in which 

 it occurs in quantities of 0-8 to 1-3 grm. in the twenty-four hours. It is 

 easily produced from creatine by boiling the latter with strong hydrochloric 

 acid, when a process of dehydration occurs. Creatine has the formula : 



NH 2 



NH = C N(CH 3 )CH 2 -COOH. 

 On dehydration it is converted into creatinine : 



NH 



NH = C N(CH 3 )CH 2 CO 



Creatinine may be obtained from urine in the following way. The urine is made 

 alkaline with milk of lime and treated with calcium chloride and filtered. The filtrate 

 is slightly acidified with acetic acid and evaporated on the water bath to a syrupy con- 

 sistence. A little sodium acetate is added and the mixture extracted with alcohol. The 

 filtered alcoholic extract is now treated with a concentrated neutral alcoholic solution of 

 zinc chloride. A crystalline precipitate of creatinine zinc chloride is produced. After 

 two days this precipitate is collected on a filter, washed with alcohol, dissolved in water, 

 and then boiled for a quarter of an hour with lead hydrate. The mixture is then filtered 

 and the filtrate evaporated to dryness. The dry residue is now extracted with cold 

 alcohol and filtered. On allowing the alcohol to evaporate, crystals of creatinine 

 separate out (Fig. 531). It gives the following tests : 



