THE EXCRETION OP URINE 513 



amount of the various salts excreted each day and the amount of water 

 absorbed by the epithelium of the tubules to account for the concentra- 

 tion in which the salts are found in the urine. In order to produce 20 

 grams of urea in 1200 c.c. of urine, 60 liters of blood-plasma fluid con- 

 taining 0.03 per cent of urea would have to be filtered through the cap- 



| 20 

 sule 1 =6000), and 5.9 liters of water returned to the blood from 



) 0.03 



the uriniferous tubules. Since the bloodflow through the kidneys is very 

 great, at least 500 liters per day, only about 13 per cent of the fluid con- 

 tained in the blood passing through the glomerulus would pass by 

 filtration through the capsule of Bowman. 



The fact that such a large amount of fluid would have to be reab- 

 sorbed from the uriniferous tubules (59 liters) is a possible a priori 

 criticism of the theory, but Cushny points out that the amount each 

 tubule would have to absorb per hour would be very small (in his ex- 

 periment on a cat amounting to less than 0.014 c.c. per hour). 



The filtration of the protein-free blood fluid through the renal capsule, 

 like that through any other membrane, depends on several factors. (1) 

 There must be a difference in the pressure between the blood and the 

 urinary filtrate. In the laboratory the pressure used in filtering is 

 usually supplied by gravity, but in the case of the filtration of the urine 

 through the capsule the force is furnished by the pressure of blood in 

 the glomerular vessels. (2) The character of the filter determines what 

 substances shall pass. The renal capsule is a membrane normally im- 

 pervious to the proteins of the blood, but pervious to the other constitu- 

 ents. Under certain conditions it loses this character. (3) The char- 

 acter of the fluid determines how readily it will filter through the mem- 

 brane. If the fluid contains a substance which can not pass through the 

 filter and w T hich exerts an osmotic pressure in opposition to the filtering 

 force, the rate of filtration as well as the amount filtered, w^ill be reduced. 



If the capsule acts as a filter it should be possible to alter the rate of 

 urine excretion by varying any of these factors, and experimentally this 

 is true. The factors can be varied in several ways. If the blood pressure 

 is raised by tying off several of the branches of the aorta, the urine is 

 appreciably increased, or if the blood pressure is decreased, as can be 

 done by compressing the renal artery by means of a screw clamp, the 

 amount of urine is decreased. In the artificially perfused kidney, the 

 fluid exuding from the ureter increases as the pressure of the perfusion 

 fluid is increased, arid decreases as the pressure is decreased. Whether 

 changes in the pressure in the blood are directly responsible for variations 

 in the rate of urine excretion, or whether they act indirectly by varying 

 the rate of the bloodflow in the kidneys, has been the subject of much 



