ROLE OF THE BLOOD-BRAIN-BARRIER IN CEREBR AL HOMEOSTASIS

By a variety of mechanisms, the cerebral endothelium isolates the extr acellular fluid space in the central nervous system from the plasma. T he combination of physical and enzymatic mechanisms which prevent macr omolecules, polar solutes, neurotransmitters, peptides, and electrolyt es from passively entering the brain has been termed the blood-brain b arrier (BBB). Specific mechanisms provide facilitated transport across the BBB and active secretion of extracellular fluid and CSF maintain homeostasis for nutrients and for cation and H+ respectively. Conseque ntly, interstitial fluid volume in the CNS does not increase when the total extracellular fluid volume is increased. Total tissue volume is sensitive to osmotic forces, while oncotic forces are relatively unimp ortant. Most anaesthetic drugs are sufficiently lipid soluble that the y enter the CNS easily by passive diffusion. Differences in the rates of CNS penetration between drugs can be predicted from their lipid sol ubility. Anaesthetic drugs have little effect on BBB permeability and their effects on brain oedema formation derive principally from their haemodynamics effects.