The blood-brain barrier consists of the brain capillary endothelial cells joined by tight junctions . Unlike somatic capillaries, brain capillaries have no fenestrations and show no pinocytosis. Astrocytic foot processes cover 85 percent of the brain capillary surface. The ability to exclude certain substances from brain interstitial space has to do not only with the anatomical capillary barriers, but also with lipid solubility and selective transcellular transport by endothelial cells. On the other hand, glucose, urea, ions and substances ionized at physiological pH cross the blood-brain barrier poorly.
Hypertonic stimuli can open transiently the endothelial junctions.
The interstitial space of the brain is separated from the ventricular CSF by the ependymal lining and from the subarachnoid CSF by the glia limitans. The glia limitans is a thick layer of astrocytic processes with an overlying basement membrane. External to it is the pia matter, a thin layer of connective tissue cells with a small amount of collagen. The ependymal barrier is far more permeable than the blood-brain barrier. Blood may be spilled into the CSF by accidental puncture of a leptomeningeal vein during entry of the LP needle.