Anoxia kills most vertebrates in minutes because energy production stops and lactic acid accumulates. The painted turtle addresses both problems. Cold water drops its metabolic rate to a fraction of the summer value, so demand is tiny, and it continues to produce energy anaerobically.
The acid problem is handled by the shell and skeleton. Carbonate minerals in the bone buffer lactic acid released into the blood, and the shell also absorbs lactate directly into its tissue. Laboratory studies have kept adult painted turtles anoxic at about 3 degrees Celsius for over four months with survival.
Hatchlings solve winter differently. Many stay in the nest through their first winter and tolerate body temperatures below freezing by supercooling, holding body fluids liquid below zero rather than allowing widespread ice to form.