Electrokinetic effects on luminal and transmural fluid flow in capillaries

The influence on fluid flow of the fixed charge on the surface of capillaries is calculated using the linearised Poisson-Boltzmann equations. The results depend strongly upon the ratio of the capillary radius to the Debye length. At physiological ionic strength, the Debye length is less than 1 nm and electrostatic effects are negligible. In particular, they can not explain the Copley-Scott Blair phenomenon in artificial capillaries. Electrostatic effects can be significant in smaller channels and it is calculated that in intercellular clefts in the capillary endothelium the apparent viscosity of the fluid may increase more than 50%. These effects can also be important in the flow in the narrow gap between a red cell and the blood capillary wall. Using the Fitzgerald-Lighthill model of this flow and parameters typical of the human microcirculation, the theory predicts that the apparent viscosity in the gap will be increased by about 5%.