blood-brain barrier

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A physiological mechanism that alters the permeability of brain capillaries, so that some substances, such as certain drugs, are prevented from entering brain tissue, while other substances are allowed to enter freely.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun anatomy A structure in the central nervous system (CNS) that keeps various substances found in the bloodstream out of the brain while allowing in the substances essential to metabolic function, e.g. oxygen.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a mechanism that creates a barrier between brain tissues and circulating blood; serves to protect the central nervous system