EXCHANGE OF SUBSTANCES THROUGH CAPILLARY WALLS 



IOO7 



through large pored artificial membranes. Thus 

 Meschia & Setnikar (250) found that less than 2 per 

 cent of the ideal osmotic potential was developed by 

 sucrose during osmotic flow through a collodion 

 membrane having pores of radius no A (i.e., when 

 the radius of the pore was approximately 25-fold 

 greater than the radius of the diffusing molecule). 

 Similarly low values for osmotic reflection coefficient 

 have been reported by Grim (125) and by Shuler 

 et al. (333) on the basis of osmotic flow through 

 uncalibrated membranes. 



Durbin (82) has recently completed a study of 

 osmotic flow caused by molecules of graded sizes 

 diffusing through calibrated porous membranes. His 

 results shows that 



-('■%.) 



where A s is the restricted pore area available to the 

 solute and A u . is the restricted pore area available to 

 the solvent as defined by equation 7.9 and figure 7.1. 

 Durbin's results indicate that during osmotic flow- 

 through artificial membranes the value of a is less 

 than o. 1 when the radius of the diffusing molecule is 

 10 per cent of the radius of the pore. 



From existing data one must therefore conclude 

 that only a small fraction of the theoretical van't 

 Hoff pressure is operative across artificial membranes 

 during restricted diffusion of small molecules through 

 the membranes. On the other hand, relatively large 

 osmotic forces have been observed during the re- 

 stricted diffusion of small molecules through biological 

 membranes under conditions involving little or no 

 net fluid movement (122, 281). Under these con- 

 ditions the osmotic reflection coefficient appears to 

 depend upon the restricted diffusion coefficient of 

 solute relative to that of the solvent. 



u w » w 



Combination of equation 7.18 with equation 7.9 

 allows numerical evaluation of a for the case of zero 

 net fluid movement when molecular radius and pore 

 radius are known. Thus, 



rapidly in the virtual absence of net fluid movement 

 (281). 



In experiments involving artificial membranes it is 

 exceedingly difficult to measure osmotic forces in the 

 absence of osmotic flow, as pointed out in section 3. 

 For this reason all investigations of osmotic reflection 

 coefficient in artificial systems have thus far invoked 

 net flow of fluid. Under these conditions the frictional 

 forces determining osmotic reflection coefficient will 

 contain hydrodynamic as well as diffusional terms as 

 emphasized in recent derivations by Ray (292) and 

 Katchalsky (169). Discussion of these derivations is 

 beyond the scope of this chapter but it seems fair to 

 say that no well-substantiated theory is yet available 

 to predict osmotic reflection coefficients as a function 

 of membrane permeability and flow rate. Since most 

 biological membranes allow the restricted passage of 

 environmental solutes, the problem remains as one of 

 the most important unsolved questions in contem- 

 porary studies of permeability. 



8. TRANSCAPILLARV MOVEMENT OF 



LIPID-INSOLUBLE MOLECULES 



The concentration gradients which provide the 

 driving force for diffusion exchange between blood 

 and tissues are normally maintained by tissue me- 

 tabolism. However, the transcapillary exchange 

 process is so efficient that normal transcapillary 

 concentration differences of small molecules would be 

 too small to be detectable by existing methods, even 

 supposing it were feasible to collect, for analysis, 

 tissue fluid from the immediate vicinity of the capillary 

 wall. From an experimental point of view it is there- 

 fore necessary to establish abnormally large trans- 

 capillary concentration ratios in order to study 

 diffusion characteristics of the capillary walls. 



Figure 8.1 summarizes data showing rates of 

 disappearance from the circulatory system of various 

 lipid-insoluble substances which distribute primarily 

 in extracellular fluid. It is evident that these sub- 

 stances leave the vascular system at rates which vary 



IT.* I- 



D '/0- &f*0&+ 2.09&-0.95&] I 



(7. 19) 



Equation 7.19 is specially applicable to capillary 

 membranes where osmotic forces can be measured 



inversely with molecular size. Disappearance from 

 plasma is accompanied by simultaneous appearance 



