Fig. 1. Dying or surviving in hypoxia. The key to anoxic brain survival is to maintain brain ATP levels. In the hypoxia-intolerant animal (A), anaerobic ATP production (glycolysis) has a limited capacity to compensate for the decline in aerobic ATP production (oxidative phosphorylation) during hypoxia. Therefore, ATP production soon falls passively with falling oxygen levels. Because this fall is not matched by a corresponding reduction in ATP use, the result is that ATP levels plummet, leading to membrane depolarization and a cascade of degenerative processes. In the anoxic survivor (B), the fall in aerobic ATP production is initially compensated for by an elevated anaerobic ATP production and subsequently matched by an orchestrated suppression of ATP use called metabolic depression. Thereby, ATP levels are maintained.