THE SECRETION OF URINE 1187 



be at least 30 mm. lower than tke pressure of the blood in the glomemli. 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 afTerentia 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. 545) there will be a continuous fall of pressure 

 from a to b. If however in the tube abc 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 6 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 



pressure 



6 



FIG. 545. 



causing the fluid to move from a to b is of two kinds. It equals %mv 2 -f- P, i. e. repre- 

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

 pressure between any two points of the tube. The total energy will diminish con- 

 tinuously from a to c, and is used in overcoming the resistance of the system. We may 

 say then that the sum of these two, namely, %mv 2 -j- P, is greater at a than b, and is 

 greater at b than c ; but as the fluid passes from the narrow tube a into the wide tube 6, 

 there is a sudden fall of its velocity and a consequent diminution of the factor %mv 2 . 

 In order to provide for a continuous fall in the total energy of the fluid, namely, %mv 2 -j- 

 P, the diminution in the factor %mv 2 must cause a corresponding increase in the factor 

 P, i. e. in the lateral pressure exercised by the fluid on the vessel wall. As the total 

 diameter of the bed of the stream in the capillaries may be twenty times that of the bed 

 in the vas afferens, the velocity of the blood in these capillaries will be only one- twentieth 

 of that in the artery and the kinetic energy of the blood only one four-hundredth. It 

 is possible therefore that the pressure exercised by the blood on the walls of the capil- 

 laries may be even greater than that in the interlobular arteries, and this effect will be 

 still further aided by the narrow diameter of the vas efferens. Although therefore the 

 pressure in the ordinary capillaries of the body is probably not greater than 20 to 30 mm. 

 Hg., the glomerular capillaries might present a pressure little inferior to that in the main 

 arteries of the body. 



The pressure in the ureter is under normal circumstances approximately 

 nil, whereas that in the glomerular capillaries is probably not more than 

 20 mm. Hg. below that in the main arteries of the body, so that there is a 

 difference of pressure on the two sides of the membrane more than sufficient 

 to cause a constant nitration of a protein-free fluid from the blood plasma 



