194 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



observed that the quantity of secretion varies not only with the pressure of 

 the blood within the glomeruli, but also with the quantity of blood flowing 

 through them. Heidenhain has insisted that it is this latter factor and not 

 the intracapillary pressure which determines the quantity of water secreted. 

 He believes that the glomerular epithelial cells possess the property of actively 

 secreting water, and that they are not simply passive filters ; that the forma- 

 tion, in other words, is not a simple mechanical process, but a more complex 

 one depending upon the living structure and properties of the epithelial cells. 

 In support of this view he quotes the fact that partial compression of the 

 renal veins quickly slows or stops altogether the flow of urine. Compression 

 of the veins should raise the pressure within the vessels of the glomeruli, and 

 upon the filtration hypothesis should increase rather than diminish the secre- 

 tion. It has been shown also that if the renal artery is compressed for a 

 short time so as to completely shut off the blood-flow to the kidney the 

 secretion is not only suspended during the closure of the arteries but for a 

 long time after the circulation is re-established. According to Tiegerstedt, 

 if the renal artery is ligated for only half a minute the activity of the 

 kidney is suspended for three-quarters of an hour. This fact is difficult to 

 understand if the glomerular epithelium is simply a filtering membrane, but 

 it is easily explicable upon the hypothesis that the epithelial cells are actively 

 concerned in the production of the water. 



Much of the recent work upon the secretion of urine tends to support 

 Heidenhain's opinion. Munk l and Senator made careful experiments upon 

 excised kidneys which were kept alive and in functional activity by an arti- 

 ficial supply of blood, and were able to show that the quantity of the secretion 

 depended less on the blood-pressure than on the rate of flow. So, numerous 

 experiments upon the action of diuretics 2 such as NaCl, KNO 3 , and caffein 

 aeern to have shown distinctly that the increased flow of blood caused by these 

 substances cannot be explained upon the filtration hypothesis, and that we 

 must suppose that they have a specific action upon the kidney-cells, particularly 

 the epithelial cells covering the glomeruli. 



We may assume, therefore, until the contrary is proved, that the larger 

 part of the water and inorganic salts of the urine is secreted at the capsular 

 end of the uriniferous tubule by a definite action of the living epithelial cells. 

 It must be borne in mind, however, that some water and probably also some 

 of the inorganic salts are secreted at other parts of the tubule along with the 

 elimination of the nitrogenous wastes. It is of interest to add that the most 

 important of the abnormal constituents of the urine under pathological con- 

 ditions, such as the albumin in albuminuria, the haemoglobin in ha3moglo- 

 binuria, and the sugar in glycosuria, seem likewise to escape from the blood 

 into the kidney tubules through the glomerular epithelium. 



Theoretical Considerations. Granting that the glomerular epithelium 



1 Virchow's Archiv fur pathologische Anatomic und Physiologic, etc., Ed. cxiv., 1888. 



2 See Von Schroeder: Archiv filr exper. Pathologic und Phannakol., Bd. xxiv. S. 85, and 

 Dreser, Ibid., 1892, Bd. xxix. S. 303. 



