The structure and function of the blood-brain barrier.

It is now clear that the phenomenon of a blood-brain barrier results from the high-resistance endothelium of cerebral vessels. The glial sheath appears to have no transport function but determines the specific characteristics of the cerebral endothelium. Among the transport mechanisms present in the endothelium is a potent sodium-potassium pump in the abluminal membrane. The endothelium probably secretes a small volume of fluid into the cerebral interstitium. Ouabain-insensitive potassium transport has been investigated in isolated cerebral capillaries. This component is very dependent on the osmolality of the medium, being markedly increased in a hypertonic medium and decreased in hypotonic conditions. This behavior may well be important in determining the net exchanges of potassium across the blood-brain barrier, which contribute to volume control of the brain in osmotic disturbances.