A high water permeability is clearly necessary in the glomerular capillaries and water permeability here is very much higher than in muscle capillaries. Normally this diffusional exchange does not represent any net flow in either direction because the water concentration on both sides of the capillary membrane is the same.įenestrations are found only in capillaries in special areas where a very high water permeability is necessary for the function of these areas. Because the total surface area of the body's capillaries is huge (6,300 m 2) and their walls are thin (1 mm), the total diffusional water flux across the capillaries in the body is very large indeed. Intercellular slits in the capillary membrane have a diameter of about 7 nm which is much larger than the 0.12 nm radius of a water molecule. diffusion across the lipid cell membranes of the endothelial cells.pores in the endothelial cells special areas where the cytoplasm is so thinned out that it produces deficiencies known as fenestrations.the intercellular gaps between the endothelial cells.The lipid composition of different cell membranes varies so the rate of fluid flow across cell membranes does vary. These factors must be included when considering diffusion across the membrane (as quantified by Fick's law of Diffusion) and they significantly counteract the the very low permeability. the surface area of the cell membrane is very large (relative to the contained volume).the concentration of water in water is extremely high. Now there are a couple of other equally important facts to consider: This partition coefficient is as expected, extremely low: about 10-6 which is 1 to a million. To express this slightly differently: The 'oil & water don't mix' idea can be quantified as the partition coefficient (ie concentration of water in the lipid phase to the concentration in the aqueous phase). This initially does not seem to be very credible based on the 'oil & water don't mix' idea BUT it is nonetheless extremely important because this pathway is available in ALL cells in the body. This refers to water crossing the lipid bilayer of the cell membrane by diffusion.
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