COMPOSITION AND CHARACTERS OF URINE 1245 



taken in with the food or absorbed from the intestines, since both these 

 bases may be re-excreted into the gut and appear as insoluble phos- 

 phates in the faeces. 



Normal human urine always contains a small amount of ammonia, 

 on an average between O6 and O8 grm. in the twenty-four hours. 

 As we have already seen, in dealing with the origin of urea in the 

 body, the quantity of ammonia in the urine is an index to the excess 

 of acids over bases which have to be excreted by this fluid. Thus it 

 is easily possible to increase the proportional amount of ammonia in 

 the urine by the administration of mineral acids. An increase of 

 the proportion of nitrogen excreted as ammonia, apart from the 

 administration of acids with the food, is an important indication of 

 the formation of abnormal acid substances in metabolism. Thus in 

 diabetes, when the last stages of fat oxidation are in default, so that 

 the oxy-fatty acids, ^-oxybutyric and aceto-acetic acids, accumulate 

 in the body, there is always a considerable rise in the ammonia of the 

 urine. 



It is usual to reckon iron among the bases which may be excreted 

 by the urine. The amount of this substance in the urine is 

 extremely small, as a rule less than 5 mg. in the day. It affords no 

 clue to the iron metabolism of the body, since the main channel of 

 excretion of this substance is the intestine. 



ORGANIC CONSTITUENTS OF THE URINE 



Almost all these constituents contain nitrogen, which in man is 

 distributed among the various urinary constituents as follows : 



Urea ..... 85-90 per cent. 



Ammonia ..... 2-4 ,, 

 Creatinine . . . . .3 ,, 



Uric acid ..... 1-3 ,, 



About 6 per cent, of the urinary nitrogen is in the form of other sub- 

 stances, such as hippuric acid, pigments, &c. 



/NH 2 

 UREA or CARBAMIDE, COC can be regarded as derived 



X NH 2 

 /OH 



from carbonic acid, C0(^ by the replacement of each OH group by 



X OH 



an NH group. It is isomeric with ammonium cyanate, NH 4 CNO. 

 If a solution of potassium cyanate and ammonium chloride be warmed 

 together and evaporated, crystals of urea may be obtained in long 

 colourless prisms (Fig. 514) without any water of crystallisation. It 



