Ultrafiltration in Living Capillaries

Abstract

This paper reviews the process of ultrafiltration through the walls of capillary blood vessels. Capillaries are 5µm to 20µm in diameter and their walls are made of a single layer of flattened endothelial cells which act as a barrier to plasma proteins but allow water and low molecular weight hydrophil ic solutes to exchange between the capillary blood and the surrounding tissues. Net fluid movements are driven by differences in hydrostatic pressure and by differences in osmotic pressure which arise from differences in concentration of the plasma proteins. The interaction between the surface coat of the endothelial cells and plasma proteins is important for the maintenance of normal hydraulic permeability and molecular selectivity of the capillary ultrafilter. Capillaries of the glomeruli of the kidney are specialised for ultrafiltration. They have high hydraulic permeabilities but the driving force for ultrafiltration is relatively low allowing filtration equilibrium to be approached or even achieved during a single transit. The relatively low pressures may contribute to the long term efficiency of the glomerular capillaries as ultrafilters and the part played by ultrafiltration in overall functioning of the kidneys.