n. a protective barrier of cells that line the capillary endings supplying blood to the brain and central nervous system. Semipermeable in nature, it prevents large molecules and harmful substances in the bloodstream from reaching the cerebrospinal fluid. However, ions and molecules of oxygen, carbon dioxide, water, and even alcohol can cross this barrier.
BLOOD-BRAIN BARRIER: “The blood-brain barrier helps prevent toxins and drugs from entering the brain directly through the capillaries that meet the bloodstream.”