Protective mechanisms during hypoxia or circulatory insufficiency

Increase blood flow and delivery of oxygen to the most active areas
of the brain during periods of hypoxia or circulatory insufficiency:

Release of oxygen by hemoglobin in
response to the metabolic end-product
carbon dioxide (the Bohr effect)

The cerebral cortex is most predictably susceptible
to damage from circulatory insufficiency or
hypoxia. The brainstem pattern of damage has
long been reported and discussed as an unusual
finding in cases of complete circulatory arrest
(Janzer & Friede 1980). The difference is not a
matter of degree, but whether oxygen is totally
lacking or only in short supply. The Bohr effect is
among the most elegant mechanisms that evolved
to maintain homeostasis in multi-cellular organisms
dependent upon aerobic metabolism.

Kelly PA et al. (1995) Enhanced cerebrovascular responsiveness to hypercapnia
following depletion of central serotonergic terminals. Journal of Cerebral Blood
Flow and Metabolism 15:706-713

Jensen FB (2004) Red blood cell pH, the Bohr effect, and other oxygenation-linked
phenomena in blood O2 and CO2 transport. Acta Physiol Scand.182:215