113 8 PHYSIOLOGY 



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 nitrate 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 

 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 constant, 

 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 the tubular cells, 

 must approximate more closely to the supposed glomerular transudate, 

 containing the same proportion of salts as the blood-plasma, the more rapidly 

 the formation of the glomerular transudate takes place : i.e. the quicker the 

 flow of urine the more nearly must its composition, reaction, and osmotic 

 pressure resemble those of the blood-serum. 



(3) The total quantity of solids excreted in any given time must be 

 increased with any increase in the urinary flow. For, whatever the activity 

 of the tubules, the glomeruli must blindly turn out a certain proportion of 

 solids with every cubic centimetre of fluid that they form. 



We may deal first with the influence of alterations in the renal blood- 

 supply on the flow of urine. Ligature of the renal vein diminishes and soon 

 stops the flow altogether. Since this procedure must cause a large rise of 

 pressure in the capillaries of the kidney, this result was regarded by Heiden- 

 hain as disproving any possibility of the glomerular process being of the 



