i in i X( l;i TION <t i i;i\i: 513 



amounl of the various salts excreted each day and the .- 1 1 1 1 < • 1 1 1 1 1 of water 

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

 tion in which the salts are found in the urine. In order i'> produce '- h| 

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

 taining 0.03 per cent of urea w.mld have to 1"' filtered through the cap- 



20 

 suit' — =6000), and 5.9 liters of water returned to the blood from 

 0.03 



tlir uriniferous tubules. Sine' the 1»1 Iflow through the kidneys is v< 



great, a1 leasl 500 liters per day, only aboul 1". per cenl of the fluid con- 

 tained in the blood passing through the glomerulus would pass by 

 filtration through the capsule of Bowman. 



The fact thai such a Large amounl of fluid would have to be reab- 

 sorbed from tlif uriniferous tubules 59 liters) is a possible a priori 

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

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

 perimenl on a cal 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, bu1 in the ease of the filtration of the urine 

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

 the glomerular 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, ruder 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 nol pass through the 

 filter and which exerts an osmotic pressure in opposition to the filtering 

 force, the rate of filtration as well as the amount filtered, will be redu 1 



If the capsule acts as a filter it should be possible to alter the n t< 

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



is true. The factors can be varied iii several ways. If the hi 1 pressure 



is raised by tying off several of the branch* rta, the urine 



appreciably increased, or if the blood pressure is d< :m 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 pressui the perfusion 



fluid is increased, and decreases as the pressure is d< d Whetl 



changes in the pressure in the Mood are directly responsible for variati 

 in the rate of uriu. tion. or whether the\ act indirectlv bv varvi 



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



