MECHANISM OF RENAL SECRETION 85 



CONCLUSIONS 



The mechanism of the renal secretion of iron, which we con- 

 ceive to be the true one, on the basis of the above-recorded 

 results, is one which permits of the actual secretion of iron from 

 the blood into the urine by the cells of the convoluted tubules. 

 This conception is therefore more in agreement with the orig- 

 inal theory of Heidenhain than with the ideas of Ludwig and 

 their modifications. That there is an actual biologic and vital 

 secretion of the salt by the cells and not a mere mechanical 

 filtration we beheve to be clear not only from the above-recorded 

 experimental results, but from the unanimously uniform results 

 of other workers, among whom Oliver (27, 88), Quincke (25), 

 Lescke (26), Gurwitch (17), and Glaevecke (43) deserve especial 

 note, as they worked with true salts. Although it is well known 

 that the kidney functions primarily as an excretory organ, 

 removing substances from the blood stream, but not elaborating 

 new ones, its conspicuous greed for oxygen is equally renowned. 

 This rapacious appetite for fresh arterial blood indicates a 

 rapid metabolic activity such as would be required by secretory 

 cells, but not by those functioning chiefly as membranes for 

 mechanical filtration. The functional dynamics of secretory 

 cells, in fact of all cells, undoubtedly rests upon a purely phys- 

 ical and chemical basis, but their structure and composition 

 are so extremely complex that the explanation of their mechan- 

 ics requires more elaborate study and understanding before we 

 can speak of simple filtration, osmosis, or adsorption in connec- 

 tion with cells. Even the recent work on organic colloidal gels 

 and sols must be interpreted as being fundamentally different 

 from the existing conditions in cells where we have a highly 

 complex and organized mixture of truly innumerable different 

 colloids and solutes, all differing both in physical and chemical 

 properties. To conclude that such an involved mixture behaves 

 identically with a preparation of a single pure colloid is errone- 

 ous, but we may conclude that it will behave similarly. 



The proximal portion of the convoluted tubule is the unit 

 which is active in secreting iron. Whether or not the distal 



