Editing Hypoxia (medicine) (section)

====Cerebral ischemia====
The brain has relatively high energy requirements, using about 20% of the oxygen under resting conditions, but low reserves, which make it specially vulnerable to hypoxia. In normal conditions, an increased demand for oxygen is easily compensated by an increased cerebral blood flow. but under conditions when there is insufficient oxygen available, increased blood flow may not be sufficient to compensate, and hypoxia can result in brain injury. A longer duration of cerebral hypoxia will generally result in larger areas of the brain being affected. The [[brainstem]], [[hippocampus]] and [[cerebral cortex]] seem to be the most vulnerable regions. Injury becomes irreversible if oxygenation is not soon restored. Most cell death is by [[necrosis]] but delayed [[apoptosis]] also occurs. In addition, presynaptic neurons release large amounts of glutamate which further increases Ca<sup>2+</sup> influx and causes catastrophic collapse in postsynaptic cells. Although it is the only way to save the tissue, reperfusion also produces reactive oxygen species and inflammatory cell infiltration, which induces further cell death. If the hypoxia is not too severe, cells can suppress some of their functions, such as protein synthesis and spontaneous electrical activity, in a process called ''[[Penumbra (medicine)|penumbra]]'', which is reversible if the oxygen supply is resumed soon enough.<ref name="Michiels 2004" />