EXCHANGE OF SUBSTANCES THROUGH CAPILLARY WALLS 



IOI7 



have been reported by Mayerson et al. (232) who 

 found that dextran fractions of mean molecular 

 weights greater than about 100,000 (a > 60 A) appear 

 in hepatic, intestinal, and cervical lymph in concen- 

 trations which are almost independent of molecular 

 size. Mayerson et al. follow Grotte in ascribing these 

 results to the presence of large capillary leaks but 

 they also mention an interesting alternative possibility 

 that very large molecules may be transported by active 

 endothelial vesiculation (pinocytosis) as described by 

 Palade (273) and Moore & Ruska (255). In calcu- 

 lating pore-size distributions Mayerson et al. do not 

 take into account sieving effects nor the fact that 

 filtration varies with the fourth power of pore radius. 

 They assume that filtration rate through pores of 

 given collective area will be the same, regardless of 

 pore radius. If 5 per cent or more of the capillary 

 pores had a radius > 200 A, as suggested by Mayerson 

 et al., then from equation 7.16 



r> ]/05(200 x id 8 ) " > 95 A 



A mean pore radius of 95 A for hydrodynamic flow 

 would lead to an improbably high value for the 

 filtration coefficient. For this reason we favor Grotte's 

 interpretation in terms of molecular sieving through 

 pores of radius 40 to 45 A combined with relatively 

 few large capillary leaks. Both interpretations are 

 subject to the criticism that lymph is not a capillary 

 ultrafiltrate and may well be modified by capillary 

 reabsorption (208), particularly at low rates of lymph 

 flow. 



A more clear-cut application of the theory of 

 molecular sieving is possible in the case of renal glo- 

 merular membranes. Figure 10.3 shows the glomeru- 

 lar clearances of several proteins and dextrans relative 

 to creatinine in the dog. The apparent differences 

 between glomerular sieving of dextran molecules and 

 proteins of equivalent molecular radius may be 

 spurious because the dextran fractions were not 

 perfectly monodisperse and it is possible that the 

 lesser degree of sieving for each nominal molecular 

 radius represents the contribution of smaller dextran 

 molecules. The data agree well with theoretical curves 

 for molecular sieving through an isoporous membrane 

 having pores 35 to 42 A in radius and a total pore 

 area per unit path length for water of 1.6 X io 5 cm 

 per g kidney (278). Substitution of these values in 

 equation 7.13 leads to filtration coefficients in the 

 range 3.5 to 5.0 X io -5 cm 5 dyne -1 sec -1 per g or 2.7 

 to 3.9 ml per min per mm Hg per 100 g kidney. 

 These values are in excellent agreement with esti- 

 mates based on hemodynamic data (339, 384). Al- 

 most identical values for renal glomerular perme- 

 ability have also been derived by Lambert and his 

 associates (194-196) from molecular sieving of hemo- 

 globin as a function of glomerular filtration rate. 



The greater permeability of renal glomerular mem- 

 branes relative to peripheral capillaries is evidently 

 due to a relatively large fractional pore area rather 

 than to large pores. Given a path length for filtra- 

 tion and diffusion of 0.5 X io~ 4 cm, the glomerular 

 pore area for passage of water would be 8 cm 2 per g 



C2/C1 



GLOMERULAR CLEARANCE 



RELATIVE TO CREATININE 



1.0 



DEXTRAN 

 FRACTIONS 



fig. 10.3. Theoretical vs. ac- 

 tual molecular sieving through 

 renal glomerular membranes 

 of dogs. Molecular sieving of 

 myoglobin, egg albumin, and 

 hemoglobin calculated as in 

 references 278 and 194. Data 

 for dextran fractions are taken 

 from Wallenius (366). 



40A 



