Publisher Summary The essential feature of hypoxia is the cessation of oxidative phosphorylation when the mitochondrial PO2 falls below the critical level. The anaerobic pathways, in particular the glycolytic pathway, then come into play. The glycolysis is initiated under hypoxic conditions by the accumulation of adenosine diphosphate (ADP) that acts on rate-limiting steps in the metabolic pathway. The anaerobic metabolism produces only one-nineteenth of the yield of adenosine triphosphate (ATP) per mole of glucose when compared with aerobic metabolism. Therefore, during hypoxia, the ATP/ADP ratio of the brain falls and there is a decline in the level of high energy compounds, including phosphocreatine. The changes are very rapid in an organ with a high metabolic rate and, in the rat brain, respiratory arrest results in a precipitous fall of all high energy compounds within a few minutes. The very similar changes occur in response to arterial hypotension. These changes rapidly block cerebral function, but organs with a lower energy requirement continue to function for a longer time.