THE SECRETION OF URINE 1271 



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 6 is of two kinds. 

 It equals \mv 2 + P, i.e. is represented 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 continuously 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^ + P, is greater at a than 6, and is greater at 6 than c ; but as the fluid 

 passes from the narrow tube a into the wide tube b there is a sudden fall of its 

 velocity and a consequent diminution of the factor %mv 2 . In order to pro- 

 vide for a continuous fall in the total energy of the fluid, namely, %mv 2 + 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 exer- 

 cised by the blood on the walls of the capillaries 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 ap- 

 proximately 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 filtration of a 

 protein-free fluid from the blood-plasma coursing through these 

 capillaries. On raising the pressure on the tubule side the filtration 

 ought to come to an end when the pressure approaches a figure which 

 is 30 to 40 mm. below that in the glomerular capillaries. A number 

 of observers have found that urinary secretion ceases when the blood 

 pressure falls to between 40 and 50 mm. Hg. The urinary secretion 

 can be stopped by raising the pressure in the tubules by means of 

 ligature of the ureter. On applying the ligature the secretion con- 

 tinues for a time until the pressure in the ureter rises up to a certain 

 point, when the secretion comes to an end. In one experiment the 

 following pressures were obtained in a dog which was secreting urine 

 copiously under the action of diuretin. Manometers were connected 

 both with the carotid artery and with the ureters so that no outflow 

 of urine was possible : 



Arterial pressure Ureter pressure 



140 .... 72 



138 .... 92 



133 . 88 



In this experiment therefore secretion came to an end with a 



