OSMOSIS. 279 



dissociation coefficient (in case of electrolytes) is known. If the 

 molecular weight is not known, as in the case of proteids, the substance 

 must be removed from the solution, and the difference in the total 

 osmotic pressure so produced estimated. 



The coefficients of diffusion must be obtained under the special 

 conditions (e.g. diffusion into serum, etc.). 



The permeability of the membrane to dissolved substances, one of 

 the most important factors, and one generally not capable of accurate 

 estimation, will not only affect the passage of water and dissolved sub- 

 stances across the membrane by osmotic action, but also the hydrostatic 

 pressure necessary to cause nitration. 



We shall here content ourselves with considering a simple but usual 

 case of absorption of a solution by blood, namely, one in which the 

 osmotic pressure of the solution is lower than that of the blood, and the 

 membrane separating the two permeable to the substance in solution, 

 and to one at least of the constituents of the blood, but impermeable to 

 others. For convenience the dissolved substance is called x, and that 

 constituent of the blood to which the membrane is permeable, y. The 

 blood, by virtue of its superior osmotic pressure, tends to take up water 

 from the solution, arid at the same time x diffuses through the membrane 

 into the blood, and y into the solution. If the blood be first supposed 

 to be stationary, a time is arrived at when the partial pressure of x and 

 y is the same on either side of the membrane ; in other words, this 

 solution of x and y is now the " solvent " in an osmotic experiment, and 

 the substances in the blood to which the membrane is impermeable are 

 the " dissolved substances." The whole of x, of y, and the water of the 

 original solution, must therefore in the end be absorbed. 1 If the blood, 

 however, is circulated, the conditions for absorption are at once improved, 

 for the diffusion of x into the blood is favoured by the fact that its 

 partial pressure in the blood is kept low by renewed supplies of blood, 

 by the stirring action of the corpuscles preventing the formation of 

 " wall layers," and by the fact that cells in other parts of the body are 

 enabled to take up the substance as it is brought round. It is also 

 evident from the above that if, as a rare case, the solution had a higher 

 osmotic pressure than the blood, provided only the membrane separating 

 the two is permeable to the dissolved substance, and impermeable to 

 some constituents of the blood, when once the solution has taken up 

 enough water from the blood, and lost enough of its dissolved substance 

 to the blood, to lower its osmotic pressure to that of the blood, the 

 process described above is gone through, and it is in the end all 

 absorbed. 



For such absorption to be carried out completely, it is evident that 

 the osmotic pressure of those constituents of the blood to which the 

 membrane and capillary wall are not permeable, must exceed the 

 pressure necessary to cause filtration across the same structures, for if 

 the available osmotic pressure on the inner side of the capillary wall is 

 less than the difference between the hydrostatic pressure on the two 

 sides of the membrane, filtration must occur, and the solution can never 

 be totally absorbed. 



The assumption is here made that the resistance to the passage of 

 fluid across the membrane is the same in both directions. It must be 



1 For this explanation to hold good, the substances in the blood to which the membrane 

 is impermeable must be in true solution; and capable therefore of exerting osmotic pressure. 



