When little or no oxygen is dissolved in the blood of most animals or humans, death quickly follows. The chemistry by which our cells generate energy to stay alive needs oxygen. When oxygen runs short, a second, temporary, energy-producing system kicks in. This system doesn't need oxygen, but it produces lactic acid that can kill living cells. Therefore, this second energy system is an emergency measure, used only when seriously starved for oxygen.

Ref: Janet M. Storey and Kenneth B. Storey. "Out Cold." Natural History, 1/92, pp. 23-25. Photo: Painted turtle, courtesy of Jmalik and licensed under the terms of the GNU Free Documentation License, Version 1.2.

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