Cerebrovascular Water and Ion Transport

The capillaries of the central nervous system (CNS) are unique among the blood vessels of the body in having tight junctions between adjacent endothelial cells, no fenestrae, and a complete pericapillary investment (the glial foot processes) between parenchymal cells and capillary endothelium. The net result of these singular properties is to transform an ordinary capillary endothelium into a formidable restriction to the passage of both solute and solvent, a blood-brain barrier (BBB). The BBB serves to passively isolate the brain interstitial microenvironment from the plasma and, by means of carrier transport systems resident on the capillary endothelial wall, to actively regulate the passage of substrates and metabolites between the blood and the brain extracellular space. Taken together, these active and passive regulatory functions of the BBB serve to maintain an optimal environment for neuronal function and for transfer of information within the brain parenchyma (Nicholson, 1980; Cserr and Bundgaard, 1984).