The blood-brain barrier: Transport across the cerebral endothelium

Abstract

The blood-brain barrier is now known to be the cerebral endothelium. Except in certain regions, this is a very tight cellular membrane with high electrical resistance, a low filtration coefficient, and low permeability to many polar solutes. It contains specific transport mechanisms for a number of metabolic substrates. The ionic composition of the interstitial fluid of brain is maintained by active transport at the endothelium, and there may well be a slow net secretory production of this fluid. The astrocytic ensheathment of cerebral capillaries probably induces and maintains the particular properties of this endothelium. It may be a route by which neuronal activity can influence blood flow or permeability or both. The permeability surface area product diffusion coefficient (PS) product of the cerebral microvessels to glucose and to other solutes may rapidly and markedly change in relation to neuronal activity and metabolism. The mechanism of the increase of PS product is unsettled, but the phenomenon probably may be explained by recruitment or flow redistribution in the microcirculation, or both.