2 7 8 



DIFFUSION, OSMOSIS, AND FILTRATION. 



in the intestine purge (sodic sulphate), while others do not (sodic 

 chloride), may in the end find its explanation in a permeability of the 

 membrane by the latter, but not by the former. A universal 

 " physiological salt solution," then, if by such a term is meant a salt 

 solution in which tissues neither lose nor take up water, and the dis- 

 solved substance of which does not enter the cells, is not a possibility ; 

 each tissue must in fact have its own "normal solution," 1 and this may 

 possibly be in some cases a solution of some other substance than a 

 salt. 



The effective osmotic pressure, therefore, exerted against membranes 

 such as those in the body which are, as a rule, partially permeable to 

 dissolved substances, is far below that measured by a semipermeable 

 membrane, and freezing-point determinations of osmotic pressures 

 (determinations which give a gauge of the full osmotic pressure as it 

 would be exerted against a semipermeable membrane), are of but 

 orienting value to the physiologist, except in cases where the per- 

 meability of the membrane to the substance in solution is known. 



The following table from Pfeffer 2 is illustrative of the diminution in 

 the estimate of the full osmotic pressure caused by substituting a 

 permeable membrane (bladder or parchment paper) for copper ferrocy- 

 anide, and it is evident that the effect is far more marked in the case of 

 the crystalloids (saltpetre and sugar) than in that of the colloid (gum). 



The pressures are in cms. of mercury. 



The conditions, then, for the interchange of water and the con- 

 stituents of solutions through membranes in the body, are evidently 

 exceedingly complex, and it is at present practically impossible to assess 

 the value of all the factors. Broadly stated, the following factors are 

 concerned : 



1. The quantitative composition of the solutions separated by the 

 membrane, and consequently the partial osmotic pressure exerted by the 

 several constituents. 



2. The coefficients of diffusion of the various constituents. 



3. The permeability of the membrane in its physiological condition 

 to the constituents. 



4. The circumstances affecting the relative concentrations of a 

 constituent on the two sides of the membrane with time, e.g. circulation 

 and stirring. 



5. The hydrostatic pressure on the two sides of the membrane. 



6. The temperature. 



The partial osmotic pressure of the constituents of a solution is 

 obtainable from a quantitative analysis, if the molecular weight and 



1 Koeppe, Arcli.f. d. ges. PhysioL, Bonn, 1897, Bd. Ixv. S. 492. 



2 "Osmotische Untersuch.," Leipzig, 1877, S. 73. 



