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ESSENTIALS OF PHYSIOLOGY. 



with the capillary pressure in the glomeruli ; and if the arterial blood 

 pressure falls below 40 to 50 mm, Hg, the flow of urine ceases. 



If the blood plasma is filtered through peritoneal membrane soaked 

 in gelatin, it is found that, when the difference of pressure on the 

 two sides of the membrane is 40 mm. Hg or more, the filtrate contains 

 the dissolved constituents of plasma with the exception of protein ; 

 with a lower difference of pressure no filtration occurs. This is due to 

 the fact that the proteins in plasma exert an osornotic pressure equal 

 to abput 25 mm. Hg, and water tends to pass back by osmosis from 

 the filtrate into the plasma. It is for this reason that the passage of 

 fluid through the glomerular wall ceases when the arterial blood pressure 

 falls below 40 mm. Hg. If the osmotic pressure of the protein is 

 diminished by decreasing the amount of protein in the plasma, for 

 instance by diluting the plasma, urine may be formed when the blood 

 pressure is considerably less than 40 mm. Hg. In hydraemic plethora 

 (p. 240), not only is the plasma more diluted, but the renal vessels are 

 dilated and the pressure in the glomeruli is raised ; and the formation 

 of urine may be extremely rapid and profuse. 



The difference of pressure on the two sides of the walls of the 

 glomeruli may be diminished either by lowering the capillary pressure 

 or by raising the ureter pressure. When the escape of urine from the 

 ureter is prevented, the formation of urine continues until the pressure 

 in the ureter is 40 to 50 mm. Hg below that in the blood-vessels, after 

 which no more urine is formed. 



We may conclude, therefore, that the amount of urine formed by 

 the glomeruli varies directly with the difference of pressure on the two 

 sides of the glomerular membrane, and that it is formed by a purely 

 physical process of filtration. An apparent exception is seen when the 

 renal vein is ligatured : the capillary pressure rises, but the flow of 



