EXCHANGE OF SUBSTANCES THROUGH CAPILLARY WALLS 



101 



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fig. 9.3. Diagram illustrating current concepts of fine structure in muscle capillaries. [From 

 Fawcett (93).] The nucleus of a single endothelial cell is shown at left. The capillary cross section 

 may be formed by a single cell rolled into a tube or may be made up of several cells. The inter- 

 endothelial region appears as a thin slit pore with direct connection from inside to outside of capil- 

 lary. The slit may be straight or slightly tortuous (inset and fig. 9.4) and is usually about 90 A wide 

 and 0.5-1 n in length. It occupies only a fraction of 1 % of the total endothelial surface. The cyto- 

 plasm contains the usual organelles, but in addition contains numerous small vesicles and inpocket- 

 ings of the surface which are characteristic of microphagocytosis or pinocytosis. Palade (273) has 

 suggested that this mechanism may be involved in the transcapillary passage of particles which 

 are too large to traverse interendothelial openings. The outer surface of the endothelium is en- 

 veloped by an amorphous basement membrane about 600 A in thickness and with histochemical 

 properties indicative of a mucopolysaccharide. The permeability properties of this membrane are 

 unknown. Dilute solutions of mucopolysaccharides, in contrast to gelatin-gels (112), may offer 

 appreciable resistance to free diffusion (266). Finally, it should be emphasized that the diagram 

 refers only to muscle capillaries. Morphological differences between capillaries in different vascular 

 beds have been reviewed by Bennett it al. (13). 



data of figure 9.2 reveals that the pore radius, r, is 

 the only unknown quantity. The numerical evalua- 

 tion of r from equation 9.3 may be carried out by 

 successive approximation or by graphical analysis. 

 For the specific example illustrated in figure 9. 1 the 

 value of r is 41 A. Similar analysis of data for NaCl, 

 urea, glucose, and sucrose leads to mean pore radii of 

 44, 43, 45, and 41 A, respectively. These results are 

 in accord with values in the range 35 to 45 A esti- 

 mated by Grotte (126) from molecular sieving of 

 dextrans in hind limb capillaries (section 10 and 

 fig. 10. 1). They are in contrast to the value of 30 A 

 estimated by Pappenheimer et al. (281) from combi- 

 nation of filtration coefficient with uncorrected 

 diffusion data and to the value of 25 A estimated by 

 Renkin & Pappenheimer (301) from uncorrected 

 restriction to diffusion. The dimensions and fractional 



surface area of aqueous transcapillary pores cor- 

 respond closely with dimensions of interendothelial 

 junctions as determined by electron microscopy. 

 Figure 9.3 summarizes pertinent aspects of pore 

 structure in muscle capillaries; details of a typical 

 interendothelial junction are illustrated in figure 9.4. 

 On the basis of present information we can compare 

 the permeability of capillaries in 100 g of muscle to 

 that of an artificial membrane, 7000 cm 2 in total 

 area, 0.5 it thick and containing 5 X io 12 uniform, 

 water-filled, cylindrical pores of radius 40 to 45 A. 

 Such an artificial membrane would have the same 

 filtration coefficient as the capillaries in 100 g muscle 

 and would restrict the diffusion of uncharged, lipid- 

 insoluble molecules to about the same degree (fig. 

 9.2). There are no reasons for supposing, however, 

 that the channels through capillary walls are either 



