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Passage of substances across the blood brain barrier

(1) Passive diffusion. Fat-soluble substances dissolve in the cell membrane and cross the barrier, eg, alcohol, nicotine, and caffeine. Water-soluble substances like penicillin have difficulty in getting through. (2) Active transport. Substances that the brain needs such as glucose and amino acids are carried across by special transport proteins. (3) Receptor-mediated transport. Molecules link up to receptors on the surface of the brain and are escorted through, eg, insulin. (Contributed by Dr. K K Jain.)