Capillaries are the sole means by which oxygen and nutrient substrates are delivered to tissues so the capillary wall acts as a size-dependent filter. The capillary wall comprises a basement membrane lined by endothelial cells.\nAlthough the brain capillaries have endothelial cells with tight junctions, in order to perform their usual function, at some stage the tight junctions must break down, temporarily.\nThe most permeable capillaries have fenestrated endothelium as in the renal glomerulus and in the third ventricle so the basement membrane is the only barrier.\nIn 1980 I proposed that basement membranse have the property of thixotropic gels and thus exhibit pressure-dependent permeability. The effects were demonstrable by increasing blood viscosity which increased the size of the proteins in the urine. The general concept was that the membrane was a gel through which water and small molecules passed without change in the membrane. But large molecules (and cells ) had to have sufficient kinetic energy to dislodge the gel units which were replaced after the molecule passed. A German time-lapse cinematograph showed a white cell push a pseudopodium through a basement membrane and the cell followed, showing the hour-glass shape as it traversed the membrane. No breach in the membrane could be demonstrated after the cell passed through the membrane, indicating its self-repair. Warfel and Hull have reported similar observations.\nIt is possible that the immune-related breaches of the BBB may relate to a viscosity-related rise in intra-capillary pressure.