Wood frogs overwinter in shallow leaf litter rather than below the frost line, so they freeze. As ice forms in the body cavity and under the skin, the liver floods the blood with glucose, and urea also accumulates. These act as cryoprotectants inside cells, so that water freezes in the spaces between cells while the cells themselves do not dehydrate fatally or rupture.
During freezing there is no heartbeat, no breathing and no measurable brain activity. Alaskan wood frogs have been recorded tolerating body temperatures below minus ten degrees Celsius and enduring repeated freeze-thaw cycles through a long winter. Thawing proceeds from the inside outward, and the heart restarts before the extremities recover.
The limits are real. Frogs survive because ice stays extracellular; if freezing is fast enough or cold enough for ice to form inside cells, the animal dies. Wood frogs are not unique in this, since several other North American frogs including spring peepers and grey tree frogs freeze-tolerate to a lesser degree, but the wood frog is the best studied and the most extreme.