Combined junction-orifice-fiber entrance layer model for capillary permeability

The recent serial section electron microscopic studies by Adamson and Michael (1993) of frog mesentery have revealed that the large pores in the junction strand of the interendothelial clefts are widely separated 150 nm wide orifice-like breaks whose gap height 20 nm is the same as the wide part of the cleft. In this paper a modified version of the model in Weinbaum et al. (1992) is first developed in which this orifice structure is explored in combination with a random or ordered fiber matrix layer that occupies a fraction of the wide part of the cleft, This basic orifice model predicts that for the measured L_p 2.0 × 10^.7cm/s/cm H₂O, to be achieved the fiber layer must be confined to a relatively narrow region at the entrance to the cleft where it serves as the primary molecular filter. The model provides a much better fit of the diffusive permeability P for intermediate size solutes between 1 and 2 nm radius than the previous model in Weinbaum et al. where the junction strand breaks were treated as finite depth circular or rectangular pores, but like the previous model significantly underestimates P for small ions. However, it is shown that if a continuous narrow slit of approximately 1.5 nm gap height is also present in the continuous part of the junction strand, small ion permeability can also be satisfied. The continuous narrow slit provides a contribution to L_p that is comparable to the widely spaced 150 nm orifices. Thus, for the narrow slit the contribution to L_p from the orifices can be as low as 1.0 × 10^.7cm/s/ cm H₂O and it is also possible to satisfy the 2.5 fold increase in permeability that occurs when the matrix is enzymatically removed from the luminal side of the cleft.