THE SECRETION OF URINE 1137 



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 2 + 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 b 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 4- 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 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 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 

 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 continues 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 difference 

 of pressure between ureter and arteries of between 40 and 50 mm. Hg. 



The absolute pressure attained within the ureter in any given experiment after liga- 

 ture of these tubes will vary with several factors. In the first place, if the minimum 

 secreting pressure is really conditioned by the colloid content of the blood-plasma, it 

 will be less the smaller the proportion of colloids in the plasma. In some experiments 

 (Magnus) a flow of urine was observed with a blood-pressure as low as 18 mm. Hg., but 

 in this case the blood was extremely dilute as the result of the continuous injection into 

 the blood-vessels of normal salt solution. Barcroft and Knowlton have shown that the 

 diuresis brought about by injection of saline (Ringer's) solution is inhibited by mixing 



