- Cerebral ischemia is when the brain is ischemic, the partial pressure of carbon dioxide (PCO2) in brain tissue increases.
- Chemoreceptors in the vasomotor center respond by increasing sympathetic outflow to the heart and blood vessels.
- Constriction of arterioles causes intense peripheral vasoconstriction and increased TPR.  Blood flow to other organs (e.g., kidneys) is significantly reduced in an attempt to preserve blood flow to the brain.
- Mean arterial pressure can increase to life-threatening levels.
- The Cushing reaction is an example of the response to cerebral ischemia.  Increases in intracranial pressure cause compression of the cerebral blood vessels, leading to cerebral ischemia and increased cerebral PCO2.  The vasomotor center directs an increase in sympathetic outflow to the heart and blood vessels, which causes a profound increase in arterial pressure.