Comparison of the capillary membrane properties determining fluid exchange in single capillaries and whole organs.

We have selected whole organ and single capillary data to examine the hypothesis that a network of fibrous molecules on the surface of the endothelial cell determines the selectivity and specific hydraulic conductivity of the walls of capillary blood vessels. Fibers with a 0.3-0.6 nm radius, occupying from 3 to 5% of the pathway volume, account for the albumin reflection coefficient and the specific hydraulic conductivity of transcapillary channels in the capillaries of frog mesentery and mammalian hindlimb, heart and lung. Similar fiber dimensions account for the selectivity of fenestrated capillaries of dog gastric wall to albumin, but they predict a specific hydraulic conductivity for this organ which is smaller than the observed value. Plasma proteins are required to maintain the integrity of the size-limiting structures in the capillary wall. The data from Ringer perfused vessels conform to the hypothesis that the fibers condense into a coarser network when protein is removed from the perfusate.