Countercurrent exchange

“Another example is found in the legs of an Arctic fox treading on snow. The paws are necessarily cold, but blood can circulate to bring nutrients to the paws without losing much heat from the body.

As the (cold) blood flows back up from the paws through the veins, it picks up heat from the blood flowing in the opposite direction, so that it returns to the torso in a warm state, allowing the fox to maintain a comfortable temperature, without losing it to the snow.

This system is so efficient that the Arctic fox does not begin to shiver until the temperature drops to −70 °C (−94 °F).”

https://en.wikipedia.org/wiki/Countercurrent_exchange