What exactly is the blood-brain barrier?
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In most of the body, the smallest blood vessels, called capillaries, are only one cell thick, and those cells are somewhat loosely joined together. So it’s pretty easy for most substances to get from the blood into the tissues of our body.
But in the brain, the capillaries are two cells thick, and the outer layer is made of special cells called **astrocytes**. The cells are very tightly joined together, and very picky about what materials are allowed to leave the blood and get into the brain tissue.
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