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For optimal function of the brain with its meticulous operations, an adequate and constant micro environment seems to be a prerequisite. This is secured by the blood-brain barrier which is impermeable to hydrophilic substances, with notable exceptions such as glucose, which cross the barrier by a mechanism of facilitated diffusion. A constant micro environment is further secured by the blood flow which is balanced to the metabolic demand of the cerebral tissue and which also contributes to the maintenance of a constant pH. During activation, blood flow and glucose consumption increase more than oxygen consumption in activated areas of the brain. The flow increase forms the physiological basis for measurement and mapping of functional activation using positron emission tomography and the changes in the metabolic pattern which has been called uncoupling of flow and oxygen metabolism is the basis for such measurements using functional magnetic resonance imaging.

http://www.ncbi.nlm.nih.gov/pubmed/12468012