1270 PHYSIOLOGY 



of the glomerular functions two opinions have been held. According 

 to the Ludwig school the process is one simply of nitration, 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 practically 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. 158) that in order to produce a filtrate containing only water and 



pressure 



* . ' "" "* 



"1 pressure 



FIG. 530. 



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 fil- 

 trate, free from protein, from the blood-plasma circulating through the 

 glomerular capillaries, the pressure of the urine in the tubules and ureter 

 must always be at least 30 mm. lower than the pressure of the blood in 

 the glomeruli. A direct determination of the latter figure is not possible. 

 The anatomical arrangements are such as to bring this pressure up to a 

 high point. Not only are the vasa afferentia very short, but the vasa 

 efferentia are only two-thirds of the diameter of the vasa afferentia. 

 Moreover the sudden increase of bed which ensues as the blood passes 

 from the vas afferens to the bundle of capillaries must itself cause a 

 rise of pressure in the latter, due to the transformation of the kinetic 

 energy of the moving fluid into the statical energy represented by 

 pressure on the walls of the vessels. 



This point can be rendered clearer by the following considerations. If fluid is 

 flowing in a tube of continuous bore db (Fig. 530) there will be a continuous fall of 

 pressure from a to b. If, however, in the tube abc the segment 6 be of much greater 

 diameter than the segments a and c, although while the fluid is at rest the pressures 

 will be equal at all points of the system, as soon as the fluid moves from a to c, 

 although there is a fall of pressure between a and c, a manometer attached to b may 



