218 URINE 



Theories of Renal Function 



The first conception of the renal mechanism emanated 

 from Bowman in 1842. Based on anatomical considera- 

 tions, it was an attempt to differentiate the functions of 

 the glomeruh from those of the tubules. Bowman sug- 

 gested that the glomeruh secreted a saline solution which 

 in passing down the tubules dissolved and separated urea 

 and uric acid from the cells of the tubules. The whole 

 process was a physical one. 



Two years later this view was combated by Ludwig, 

 who believed that from the glomeruli appeared a solution 

 which consisted of plasma minus the proteins. The 

 function of the tubules was to concentrate this fluid. 

 Ludwg was at first emphatic in declaring both processes 

 to be purely physical, but later, discovering that the con- 

 stituents of the urine differed quantitatively from those of 

 the blood, he withdrew from this position and was ready 

 to admit some power of selection on the part of the cells. 



In 1874 Bowman's theory, which had been more or less 

 echpsed by Ludwig's, was revived in a modified form by 

 Heidenhain. 



Heidenhain, hke Bowman, located the secretion of the 

 water and salts in the glomeruh, and that of the other 

 sohds in the tubules. He differed from Bowman in very 

 definitely attributing both processes to the selective power 

 of the urinary cells. 



It will be seen that the controversy is a double one. 

 First, is the separation of urine from the blood due to 

 passive filtration or to active secretion ? Secondly, is 

 the function of the tubules to secrete urinary constituents 

 or to absorb water? The one point on which there is 

 universal agreement is that the urine becomes in some 

 way concentrated as it passes down the tubules. But it 

 must be emphasised that the differentiation of the glomer- 

 ular from the tubular function is based entirely on the 

 histological appearance of these structures. As long as we 

 are ignorant of the nature of the glomerular fluid, concentration 

 in the tubules must remain an assumption. 



