Ask the doctor: What is the blood-brain barrier?

Q. What is the blood-brain barrier? I've heard that it may have something to do with psychiatric disorders. Is that true?

A. The blood-brain barrier is an inborn biological structure that protects the brain from potentially harmful substances circulating in the bloodstream. Although the word "barrier" may conjure up an image of a concrete highway divider or a brick wall, this barrier is not actually impermeable. Consisting of a tightly packed cell wall that is semipermeable, it is more of a border than a barrier, one that allows some substances to pass into the brain while blocking others.

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