The gel hypothesis applied to the rat renal capillary membranes: a review

In the gel model for the glomerular (and peritubular) capillary membrane, the integrity of the membrane is supposed to result from the fluid reabsorption induced by the osmotic action of the counter-ions attracted to negative fixed charges, increasing the gel pressure such that it becomes the same as in the capillaries. From this point on, the gel will be unaffected by the high capillary pressure. The same fluid reabsorption will also suspend the fibrils in the matrix such that they form a series of grids composed of, for example, horizontal fibrils spaced similarly from one another. The model thereby explains the well-known phenomenon of a uniform 'pore' size, although slits rather than pores constitute the transport routes. The model also explains the fact that the plasma proteins are free to move in the membrane matrix, which is the consequence of a recent finding that a major restriction to albumin is offered by a unique protein, nephrin, located between the podocytes in Bowman's space cells. A large molecule, which may become trapped in a slit between two fibrils, will thus push out the positive counter-ions whereby the charges become free and hence repel one another, widening the slit such that the molecule is free to move in any direction. It is furthermore concluded that the restriction to proteins is also dependent on the width of the slits closest to plasma.