The blood brain barrier(BBB) is the name given to a group of structures that protect brain neurons. Its function is to maintain a constantly friendly chemical environment around brain tissue. The relevant structures include the endothelial cells of brain capillaries, the basement membrane of the endothelium, and the feet of astrocytes. The selectively permeable BBB membrane allows certain blood solutes to diffuse through capillary walls, but it blocks some substances from entering the CSF. It also pumps out of the CSF certain ions and molecules inimical to optimal functioning of neurons.
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The blood brain barrier is a structure ( group of structures) that operates to protect sensitive neurons against potentially harmful chemical changes in the brain interstitial fluid. The capillary endothelial cells , the capillary wall basement membrane, and the astrocytes( glia) that are attached to capillaries, all contribute to the operation of the BBB selective barrier. However, the essential operational component of the BBB is the tight junctions of the capillary endothelium. The other two structures make the following contributions: the astrocytes stimulate the endothelial cells to form the tight junctions; the basement membrane enhances the effectiveness of the barrier. The BBB is necessary because neurons are very sensitive to ions, electrolytes, hormones and neurotransmitters dissolved in blood plasma. Unusually high levels of these substances can cause the excitable neurons to become overloaded and malfunction. The tight junctions of the capillary endothelial cells deploy a special selectively permeable surface around most--but not all- of the neurons of the brain. The barrier allows glucose , essential amino acids, and electrolytes to pass through by facilitated diffusion. Some proteins, metabolic wastes, some toxic substances and some drugs are blocked. The BBB does not constitute a barrier to fats, fatty acids, oxygen, carbon-dioxide, fat soluble substances like alcohol, nicotine, some drugs and some anesthetics. In addition, like plasma membranes, the BBB is able to pump certain solutes like potassium ions, and non-essential amino acids from the interstitial fluid (CSF).. In spite of its importance, the BBB is absent or more permeable in some areas of the brain like the hypothalamus and the vomiting center. This facilitates the monitoring of plasma for toxic substances. The brains of the newborn and premature infants are more susceptible to damage by toxins, because in these brains the BBB is incomplete .