1272 PHYSIOLOGY 



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 ligature 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 considerably 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 them- 

 selves, and the observed difference of pressure during secretion is 

 greater than this osmotic pressure, we are justified in concluding, pro- 

 visionally at any rate, that the mechanical factors present at the upper 

 end of the urinary tubule are sufficient to account for the production 

 of a glomerular transudate free from protein, but containing the same 

 proportion of water and salts as the blood- plasma circulating through 

 the capillaries. 



If the process occurring in the glomeruli is simply one of filtration, 

 three conditions must be realised : 



(1) The amount of filtrate, so long as the ureter pressure is con- 

 stant, must depend on the pressure and rate of flow of the blood in 

 the; glomerular capillaries, and must fall or rise with the latter. 



(2) The constitution of the fully formed urine as it appears in the 

 ureters, after modification by addition or subtraction on the part of 



