Transport pathways for fluid and large molecules in microvascular endothelium of the dog's paw

Solvent drag coefficients (σ_f) and permeability surface area products (PS) for six plasma proteins and Dextran 110, determined in the preceding publication, were analyzed according to irreversible thermodynamic and hydrodynamic principles. The relation of σ_f to molecular size indicates that endothelial pathways admitting these molecules are not the only hydraulically conductive pathways present. At least 81.5% of the volume flow must be assigned to routes essentially impermeable to serum albumin and larger molecules. Presumably, these are the cell membranes and small pore system. The convective pathway for large molecules may represent openings between endothelial cells or fused chains of cytoplasmic vesicles forming transitory channels. Calculated PS values for large molecules in these channels are no larger than one-fourth the measured values. The residual PS must be attributed to nonhydraulically conductive pathways. If these are supposed to be nonfused endothelial vesicles, their turnover rate is estimated to be 0.4 × 10^-4 ml/sec per 24-g paw, accounting for three-fourths to five-sixths of the total dissipative solute flux. Due to the composite (parallel pathway) structure of the microvascular endothelium, σ_f for the whole membrane does not appear to be the same as the osmotic reflection coefficient, (σ_d), even though the two may be equal for individual membrane pathways (Onsager reciprocity). Over the range of experimental pressures and flows, σ_f for total plasma proteins is 0.83, while σ_d is probably 0.89 or higher. The relation between the two σs is sensitive to the microscopic disposition of osmotic and hydrostatic forces in relation to individual transport paths.