The human brain represents about 2% of the body weight yet it accounts for approximately 20%
of aerobic metabolism. This high dependency on energy-consuming processes is mainly caused by
the active transport of ions which is necessary to compensate for the transmembrane ion
currents that are part ofthe complex signaling processes in the brain. Ninety-five percent
ofthe brain's ATP is derived from mitochondrial oxidative phosphorylation. Since that organ' s
storage capacity for oxygen is minimal any interruption of oxygen delivery to brain cells
willlead to changes in membrane excitability and there fore to disruption of neuronal
signaling within seconds. It seems that mamma lian brain is especially vulnerable to such an
interruption since oxygen deprivation leads to activation of ion channel mechanisms in neurons
that impair their communications. Thus the function of the brain as a coordinator of vital
homeostatic reflexes and complex body reactions to external challenges depends critically on
the rate of oxygen delivery and oxygen consumption. Oxygen delivery depends on two variables
described in the Fick relation ship: volume flow rate ofblood and the arterial oxygen content.
A reduction in either of these two variables will have serious effects on vital brain func
tions. Reduction of arterial blood flow to the brain can be caused by cardiac arrest shock
carotid occlusion Of hypotension (global ischemia). Oxygen content is progressively decreased
in asphyxia (including drowning).
