1186 . PHYSIOLOGY 



The abandonment of Lud wig's view as to. the mechanism of the concentra- 

 tion does not however place his theory out of court. The question will 

 still have to be discussed whether the chief object of the tubules is the con- 

 centration of the fluid produced in the glomeruli, or whether they add to this 

 fluid by a further secretory process, or whether they may not possibly 

 possess both functions and in their various parts alter the fluid flowing 

 through them either by addition or by withdrawal of water or dissolved 

 constituents. The common point in the two theories is the sharp distinc- 

 tion which is drawn between the nature of the glomerular activity and the 

 nature of the activity of the tubules. The questions which we have to 

 decide by experiment are : 



(1) The nature of the glomerular activity and the conditions which 

 determine the amount of fluid formed by the glomeruli, and especially 

 whether the energy required for the formation of the glomerular fluid is 

 furnished by the heart through the blood pressure within the capillaries or 

 by the endothelium covering these capillaries. 



(2) The function of the tubules, whether they secrete or absorb, and 

 what part is played in these processes by the various segments of the tubules, 

 which differ so widely in their histological characters. 



FUNCTIONS OF THE GLOMERULI 



It is generally assumed, as the best explanation of known facts with 

 regard to the secretion of urine, that a watery exudation free from protein is 

 formed in the glomeruli, and that this becomes concentrated on its way 

 through the tubules, either by the absorption of water and certain salts or by 

 the secretion and addition of urea, 'uric acid, etc. as well as such salts as 

 acid phosphates. As to the nature of the glomerular functions two opinions 

 have been held. According to the Lud wig school, the process is one simply 

 of filtration, in which, under the pressure of the blood in the glomerular 

 capillaries, the water and crystalloid constituents of the plasma are filtered 

 through the glomerular epithelium, leaving behind the protein constituents. 

 According to Heidenhain, the process cannot be regarded as one simply of 

 filtration, but involves the secretory activity of the glomerular epithelium. 

 If the glomerular urine is a filtrate, it must resemble blood plasma in practi- 

 cally all particulars except its protein content, since the blood pressure, which 

 is the only force causing filtration, is too small to effect any appreciable 

 separation of salts. On the other hand, a certain minimum difference of 

 pressure between the two sides of the membrane must be present in order to 

 separate the colloids from the other constituents of the plasma. We have 

 seen in Chapter iv (p. 141) that, in order to produce a filtrate containing only 

 water and salts from serum, a minimum difference of pressure of 30 mm. Hg. 

 is necessary, corresponding to the osmotic pressure of the colloidal con- 

 stituents of the blood plasma or serum. Thus in order to produce a filtrate, 

 free from protein, from the blood plasma circulating through the glomerular 

 capillaries, the pressure of the urine in the tubules and ureter must always 



