400 MANUAL OF PHYSIOLOGY, 



of the urea, and possibly some other solid constituents of the 

 urine, are selected from the blood by the epithelial cells of the 

 convoluted tubules, that the fluid part of the blood escapes at 

 the glomerulus, and flows along the varied and circuitous route 

 of the tubules, carrying with it the matters poured into the tubes 

 by the cells, and that in some part of the tubules the dilute 

 filtrate loses much of its water and all its albumin. 



CHEMICAL COMPOSITION OF URINE. 



The percentage of the various materials in urine varies as the 

 secretion differs in strength, as mentioned, but on an average it 

 may be said to contain about 4 per cent, of solids and 96 per 

 cent, water. 



The following are the more important solid matters : 

 Urea is the most important, and at the same time most abund- 

 ant solid constituent, commonly forming about 2 per cent, of the 

 urine. It is regarded as the chief end product of the oxidation 

 of the nitrogenous matter in the body, so that the amount ex- 

 creted per diem gives us the best estimate of the amount of 

 chemical change taking place in the tissues. It is readily soluble 

 in alcohol and water, but insoluble in ether. It forms acicular 

 crystals with a silky lustre. From a chemical point of view it 

 may be regarded as the diamide of carbonic acid, with the 



fNH CO) 



formula CO \ -vrtr 2 ' or H 2 > N 2 . It is isomeric with ammonium 

 H 2 j 



cyanate ^-TT [ O, from which it was first prepared artificially. 



It is also isomeric with the amide of carbamic acid, with which 

 it is considered by some to be identical. 



On exposure to the air bacteria develop in the urine, and, act- 

 ing as a ferment, change the urea into ammonium carbonate, two 

 molecules of water being at the same time taken up, thus : 



CO(NH 2 ) 2 + 2H 2 = (NH 4 ) 2 CO 3 . 



This gives rise to a change in the reaction of the urine, which 

 after a time becomes, increasingly alkaline, and the change is 

 commonly spoken of as the alkaline fermentation of the urine. 



