On the Intrinsic Control of Capillary Permeability

Abstract

The permeability of the renal capillary membranes is suggested to be controlled not only by external forces, but also by the prevailing Starling pressures, in the sense that a large net driving force is followed by a large hydraulic resistance, and vice versa. This means that, in spite of variations of the net driving force, the fluid transfer may remain fairly constant—in other words an autoregulation of the membrane permeability is suggested. The need for such a mechanism would seem obvious, i.e., since otherwise even small changes in glomerular filtration and/or peritubular capillary uptake might lead to drastic changes in the urinary output. The hypothesis has its basis in a new model, in which it is supposed that a flexible gel/fiber-matrix, rather than a rigid porous membrane, constitutes the membrane. The size, shape, and permeability characteristics of such a membrane will be the result of the balance between, on the one hand, the above mentioned physical forces and, on the other, the mechanics of the gel/fiber structure. Here, the tension of the fibers acts to restrict expansion, whereas membrane-fixed negative charges, via the resulting electroosmotic pressure and balancing hydrostatic pressure, act to resist compression. Regarding the intra-membranous net driving force, the hydrostatic pressure gradient valid in the Starling model will be replaced by an electroosmotic net driving force, i.e., where the magnitude and direction of the force is governed by the electric field. A gel/fiber-matrix membrane can furthermore be predicted to possess a self-rinsing ability, a feature typical of, e.g., the glomerular capillary membrane. Regarding external control of the membrane permeability, all factors which affect the isoelectric point of the structure carrying the charges can be predicted to govern the membrane characteristics.