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The blood-brain barrier is not an absolute barrier to the passage of toxic agents into the CNS. Instead, it represents a site that is less permeable than are most other areas of the body. Nevertheless, many poisons do not readily enter the brain in appreciable quantities because of this barrier. There are four major anatomic and physiological reasons why some toxicants do not readily enter the CNS. First, the capillary endothelial cells of the CNS are tightly joined, leaving few or no pores between the cells. Second, the brain capillary endothelial cells contain an ATP-dependent transporter, the multidrug-resistant (MDR) protein that transports some chemicals into the blood. Third, the capillaries in the CNS are to a large extent surrounded by glial cell processes (astrocytes).