Capillary permeability and interstitium

Abstract

With the newly developed technique for studying solute transport across capillary walls in single capillarries by means of microelectrodes (Crone et al. 1978, Crone and Christensen, 1981) it has become possible to distinguish between diffusion across the capillary wall and diffusion in the interstitium.

When a high potassium solution is placed within a closed capillary segment in the frog’s mesentery an outward diffusion starts immediately. With potassium-sensitive microelectrodes within and just outside the capillary it is possible to study the emptying of excess potassium from the closed segment. There is a clear jump in concentration across the capillary wall, reflecting the diffusion hindrance in the endothelium and clearly showing that diffusion in the interstitium is faster than in the wall. The diffusion coefficient of potassium in the interstitium is about 40 % of the free diffusion coefficient and thus the extravascular tissue has a delaying influence upon the spread of solutes.

It is possible that in fenestrated capillaries transport across the capillary wall is so fast that the diffusion velocity in the interstitium becomes rate-limiting, but it is not very likely. The principal reason for the diffusion delay in the capillary wall is the presence of endothelial cells, reducing the available surface area for passage of hydrophilic solutes to about 1/10.000 of the total surface area.

Although the investigations have dealt with small solutes, it is likely that the conclusions also hold for macromolecules.