262 PEACTICAL PHYSIOLOGY 



Creatinin possesses, to a certain extent, the power of reducing 

 metallic oxides in alkaline solution, and this must be remembered as a 

 possible source of fallacy in testing for dextrose. 



Its significance in metabolism has been described on p. 252, where it 

 was pointed out that it comes from two sources, and that it appears 

 probable, that the exogenous portion undergoes a different metabolism 

 from the endogenous, the former having certainly nothing to do with 

 the metabolism of urea, the latter probably being an intermediate body, 

 which afterwards becomes broken down into two moieties, one con- 

 taining most of the carbon and the other an ammonia derivative. The 

 latter then becomes transformed into urea. 



The Free Ammonia. This is determined by placing a measured 

 quantity of urine, made alkaline with lime-water, under a bell-jar in 



which is a flat dish containing a measured quantity of y^H 2 S0 4 . 



The alkali slowly expels the free ammonia, which is absorbed by the 

 acid, and can be determined by titration. As mentioned on p. 255, the 

 urinary ammonia is increased in certain liver diseases. It is also 

 increased after the administration of certain ammonia salts (see 

 Advanced Course). 



CHAPTEE XVIII. 

 THE INORGANIC SALTS OF URINE. URINARY DEPOSITS. 



Chlorides. The chief base is sodium. 



EXPERIMENT I. Add to urine a few drops of nitric acid, and then a 

 solution of silver nitrate. A white precipitate (AgCl) is produced 

 soluble in ammonia. The nitric acid prevents the phosphates being 

 precipitated by the silver nitrate. 



Chlorides are derived mainly from the food, and they have, there- 

 fore, very little significance in metabolism. The percentage of sodium 

 chloride in the blood is a very constant one, and, if it should rise above 

 its normal level (as by taking much common salt in the diet), the 

 excess is at once drained off in the urine. In certain diseases where 

 effusions occur into the tissues (as in pneumonia or pleurisy) the per- 

 centage in the blood falls below normal, and less is excreted in the 

 urine, but afterwards, when the effusion is being reabsorbed into the 

 blood, the percentage again rises and an excess appears in the urine. 



Phosphates. These are mostly alkaline phosphates (NaH.,P0 4 , 

 Na 2 HP0 4 and similar salts of potassium), but there are also earthy 

 phosphates (i.e. of calcium and magnesium). 



