1188 PHYSIOLOGY 



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. Hg. 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 

 with the saline fluid colloids, such as gelatin and gum, which possess an osmotic pressure. 

 Colloids such as starch, with no measurable osmotic pressure, have no such effect. 



On the other hand, the ureters, or at any rate the urinary tubules, cannot be regarded 

 as absolutely water-tight. Not only are the cells of these tubules capable of taking up 

 fluid, but it is probable that at high pressures a certain amount of actual filtration takes 

 place between these cells. This process of reabsorption will tend to diminish the actual 

 pressure of the fluid in the ureters, so that the secretion of urine may apparently come 

 to a standstill when there is still a difference of pressure between blood and urine con- 

 siderably over 50 mm. Hg. Under such circumstances the ureter pressure will be higher, 

 and the difference of pressure between urine and blood less, the more rapid the formation 

 of urine by the glomeruli. In a number of experiments by V. E. Henderson, it was 

 found that the figure B.P. U.P. tended to approximate 40 mm. Hg. the more rapid 

 the secretion of urine was. With a slow secretion the flow of urine apparently ceased 

 when there was as much as 80 mm. Hg. difference of pressure on the two sides of the 

 glomerular membrane. 



We may conclude that, for the production of any urine by the kidney, 

 a certain minimum difference of pressure is necessary between the blood in 

 the glomeruli and the urine in the tubule, and that this difference becomes 

 less the smaller the protein content of the blood. Since the only work 

 required in the formation of a protein-free filtrate from the blood is that due 

 to the osmotic pressure of the proteins themselves, and the observed difference 

 of pressure during secretion is greater than this osmotic pressure, we are 

 justified in concluding, provisionally at any rate, that the mechanical factors 



