C FT AFTER XX 



THE SECRETION OF URINE 



THE two facts which strike us at once when considering how the 

 kidney produces urine from the blood supplied to it, are the differ- 

 ences in chemical composition between the two fluids and the 

 extraordinary structural complexity of the kidney tubule. The 

 main chemical difference between blood plasma and urine is in 

 the percentage composition of the two fluids, and not in the 

 presence or absence of substances special to each. Thus the 

 urea, sugar, and other crystalloids of the blood are found also in 

 the urine, but in wholly different percentage quantities. In fact, 

 apart from percentage composition the differences between the 

 two fluids are few. They differ, however, strikingly in reaction, 

 in the absence of the blood proteids from the urine and in the 

 presence in it of hippuric acid. The power of the kidney to 

 secrete acid urine from the alkaline blood is an important point 

 in any theory of urinary secretion, and will be considered later. 

 The ability of the kidney to keep back the blood proteids is in 

 health apparently perfect. Minute traces of proteid may be found 

 in healthy urine, but it is not one of the blood proteids, and is 

 probably derived from the urinary passages. On the other hand, 

 structural and circulatory changes in the kidney soon cause one 

 or more of the blood proteids to appear in the urine. Such 

 albuminuria is of the greatest medical importance ; but, until we 

 know more of the mechanism by which the plasma proteids are 

 kept back by the normal kidney, it is impossible to understand 

 the true significance of many forms of albuminuria. Hippuric 

 acid is interesting as being the only urinary constituent which is 

 known to be manufactured in the kidney. We know that the 

 kidney possesses a ferment which can synthesise benzoic acid and 

 glycin into hippuric acid, but, as we do not know the site in the 

 kidney of this synthesis, the fact does not help us to formulate a 

 theory of urinary secretion. Structurally the kidney bears little 



