Meaning of blood-brain barrier

blood'-brain' bar"rier

Pronunciation: (blud'brān'), [key]
— Physiol. Physiol.
  1. a layer of tightly packed cells that make up the walls of brain capillaries and prevent substances in the blood from diffusing freely into the brain: passage across the cell membranes is determined by solubility in the lipid bilayer or recognition by a transport molecule.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.