Turns out it's the arrangement of blood vessels in a dog's paws that help keep them warm no matter how brutal the winter gets. Animal expert Hiroyoshi Ninomiya of Yamazaki Gakuen University in Tokyo led a team in photographing the paws of beagles with a scanning electron microscope. The powerful SEM shots revealed that dogs keep their veins and their arteries close together in their paws, creating a counter-currentheat exchange system. In layman's terms, that means that the warm blood in the arteries flows close enough to the veins to warm them. Body heat is therefore preserved all the way down to the tips of the dog's toes.
Dogs also have the ability to open or close their blood vessels as temperatures rise or drop, meaning that their feet are as well equipped to handle the hot pavement in the summer heat as they are the winter freezes. That's why you can walk your pets on roads that would scorch your own feet in about three seconds. Your blood vessels can't narrow in the same way theirs can, so you can't keep the heat on the ground where it belongs.
Other animals that evolved in colder climates have counter-current heat exchangers in their extremities as well. Marine critters like penguins, whales, and seals all have similar systems in place to keep their feet and flippers warm even when it's about as cold as earth gets out in the arctic waters.
Ninomiya wants to conduct a similar study on the paws of cats next. Felines evolved in warmer climates than canines, so they may not have exactly the same system of heat transferal in place. Still, plenty of cats rough it outside through the winter months, so there might be something similar going on in there.