A mechanism that creates a barrier between brain tissues and circulating blood; serves to protect the central nervous system WordNet 3.0 & Open English WordNet

The blood–brain barrier is a highly selective semipermeable border of endothelial cells that regulates the transfer of solutes and chemicals between the circulatory system and the central nervous system, thus protecting the brain from harmful or unwanted substances in the blood. Wikipedia

A structure that protects the brain from chemicals in the blood Wikipedia Disambiguation

A separation of circulating blood from the brain extracellular fluid (BECF) in the central nervous system (CNS). OmegaWiki

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. Wiktionary

Structure in the central nervous system. Wiktionary (translation)