52 MEMBRANE PERMEABILITY AND DRUG ACTION 



can in principle be derived if the drug is constructed to 

 have permeabiHty properties which resemble those of 

 substances which readily penetrate into a parasitic cell, 

 but which penetrate much less readily into the cells of 

 the host. 



Until recently, however, very few attempts have been 

 made, and these unsuccessfully, deliberately to modify 

 the structure of drugs so as to exploit the permeability 

 characteristics of different types of cell. The main 

 reason for this is that there was no general quantitative 

 theory correlating permeability of cells with the struc- 

 ture of penetrating substances. Without such a quanti- 

 tative theory it is really impossible to make much 

 headway. But recently theoretical studies have been 

 made which enable us to calculate the relative permeabil- 

 ities of cells to different molecular structures, to a first 

 approximation. It is convenient to divide the cases which 

 are met in practice into five groups. 



The first group is that where permeation occurs by 

 bulk flow of fluid medium. As an instance of this we may 

 take the formation of the glomerular filtrate in the kidney 

 of mammals. Here all the crystalloical constituents of the 

 blood are filtered off through a membrane, the pore 

 size of which is very much larger than the diameter of 

 a crystalloid molecule. In such cases all the molecules 

 of a crystalloid character are carried along by the bulk 

 flow of the fluid medium. Consequently the only force 

 available to discriminate between different molecular 



