1136 PHYSIOLOGY 



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

 have been held. According to the Ludwig 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 practic- 

 ally 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. 143) 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 

 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 ab (Fig. 532) there will be a continuous fall of pressure 



b 



r--- 



""! pressure """". ^ 



pressure 



b 

 FIG. 532. 



from a to b. If, however, in the tube dbc the segment b 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 show an actual greater 

 pressure than a manometer inserted at a. Fluid is flowing from a place of lower to a 

 place of higher pressure. The apparent paradox is due to the fact that the energy 

 causing the fluid to move from a to b is of two kinds. It equals %mv z + P, i.e. represented 

 by the kinetic energy of the moving mass of fluid as well as the difference of pressure 



