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The way the blood-brain barrier is constructed, it can prevent certain substances from crossing the barrier into the brain if their molecules are too large; in addition, the capillaries contain tight junctions around them, which are not present in capillaries of the normal circulatory system, so this acts as a shield. Endothelial cells prevent the crossing over of microscopic substances like bacteria, into the cerebrospinal fluid. The cells of the barrier, however, are able to actively transport metabolic products such as glucose across it, with certain proteins.
The cells of the CNS are bathed in a fluid (extracellular fluid--ECF) that differs from that which serves as the extracellular fluid of the cells in the rest of the body. The fluid that leaves the capillaries in the brain contains far less protein than normal because of the blood-brain barrier, therefore, this barrier creates problems in medicine, as it prevents many therapeutic drugs from reaching the brain. Certain medications have been formulated to enable them to cross the blood-brain barrier, in order to help people with conditions such as anxiety, depression, alzheimers, and other conditions which require the medication to act upon brain cells in order to be effective.
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