CH. XXXVI.] 



THE WORK OF THE KIDNEY. 



543 



M 



The measurement of the work done by the kidney depends 

 upon a determination of the respective osmotic pressures of the 

 urine and blood plasma. 



If a bladder containing strong salt solution is placed in a vessel 

 of distilled water, water passes into the bladder by osmosis, so that 

 the bladder is swollen, and a manometer connected with its interior 

 will show a rise of pressure (osmotic pressure). But the total rise 

 of pressure cannot be measured in this way for two reasons : 

 (i) because the salt diffuses out as the 

 water diffuses in; and (2) because the 

 membrane of the bladder leaks; that 

 is, permits of nitration when the pres- 

 sure within it has attained a certain 

 height. 



It is therefore necessary to use a 

 membrane which will not allow salt to 

 pass out either by dialysis or nitration, 

 though it will let the water pass in. 

 Such membranes are called semi- 

 permeable membranes, and one of the 

 best of these is ferrocyanide of copper. 

 This may be made by taking a cell of 

 porous earthenware and washing it out 

 first with copper sulphate and then 

 with potassium ferrocyanide. An in- 

 soluble precipitate of copper ferro- 

 cyiinide is* thus deposited in the pores 

 of the earthenware. 



If such a cell is arranged as in 

 fig. 428, and filled with a i per cent, 

 solution of sodium chloride, water 

 diffuses in, till the pressure registered 

 by the manometer reaches the enormous 

 height of 5000 millim. of mercury. 

 If the pressure in the cell is in- 

 creased beyond this artificially, water 

 will be pressed through the semi-permeable walls of the cell and 

 the solution will become more concentrated. 



In other words, in order to make a solution of sodium 

 chloride of greater concentration than i per cent., a pressure 

 greater than 5000 mm. of mercury must be employed. The 

 osmotic pressure exerted by a 2 per cent, solution would be twice 

 as great. 



It is, moreover, found that the osmotic pressures of various 



Fig. 428. A, outer vessel, con- 

 taining: distilled water ; B, 

 inner semi-permeable vessel 

 containing i per cent, salt 

 solution ; M, mercurial 

 manometer. (After Star- 

 ling.) 



