SPECULATIONS ON FUNCTIONS OF THE BLOOD-BRAIN BARRIER

Abstract

Publisher Summary The blood–brain barrier (BBB) refers to the selective permeability of the capillaries of the central nervous system (CNS). In the entire body, about one-fifth of the tissue volume is extracellular fluid (ECF), and of this, about one-fifth is pumped through permeable microvasculature to keep the entire ECF uniform in composition. In the general body, the major routes of exchange through the capillary wall appear to be extracellular via intercellular clefts and pinocytosis. There is an agreement that these nonspecific routes are virtually absent in the brain and that almost all exchange of substances through the walls of brain capillaries seems to be transcellular, with the substances passing directly through the plasma membranes and cytoplasm of the brain capillary endothelial cell. Whether transcellular transport also takes place in general capillaries is not clear. Some substances such as blood gases readily penetrate the general capillary cells through selective exchange, but this cannot be seen against the background of the large nonspecific extracellular exchange available to all small molecules.