A hydrodynamic description of the osmotic reflection coefficient with application to the pore theory of transcapillary exchange.

Abstract An expression for the osmotic reflection coefficient (σ) has been derived starting from an expression for the force on a sphere, radius a, moving through a water filled cylinder, radius R, assuming that solute molecules can be considered hard, inert spheres, and the pathway through the membranes is a straight cylindrical pore. The factor determining the magnitude of σ is the velocity of the sphere relative to the maximum velocity of the fluid which carries it, and not the virtual area available for the diffusion of the molecule during ultrafiltration (Asf). The relation between σ and a R is significantly different from the expression obtained by Durbin (1960) : σ = 1.0 − A sf A wf . When the new description of σ was used to re-examine data previously interpreted using Durbin's relation, it was found that the equivalent pore radius compatible with the measured reflection coefficients of Vargas and Johnson (1964) was 20 A, while the permeability data of Pappenheimer et al. gave a value of 1000 A. When the latter data was analysed using Perl's (1971) model, it was found that the proportion of water flux passing via the cells increases with increasing solute size. Do water and inert solutes use the same pathway to cross the capillary wall?