Scientists at Tokyo’s Yamazaki Gakuen University wondered why dogs do not seem to feel the cold in their paws, even though the paws have less insulating fur than their trunks. The paws have pads containing a high fat content, which freezes less easily than other tissues, but they also have a high surface area-to-volume ratio, which means they should lose heat easily.
In humans exposed to frigid temperatures, vasoconstriction occurs in the extremities to reduce the blood flow and resultant heat loss, and ensure the blood returning to the rest of the body does not cool too much.
The research team, led by Dr. Hiroyoshi Ninomiya, used a scanning electron microscope to study the paws of four adult dogs, and discovered that the arteries supplying blood to the pads had networks of numerous small veins, or venules, closely associated with them, and that the system essentially acts as a counter-current heat exchanger.
When warm blood arrives in the paws via the arteries, heat is transferred to the venules closely associated with the arteries, thus ensuring the blood has been warmed up before it returns to the rest of the body.
The counter-current heat exchange system prevents the body cooling and ensures the paw temperature stays within reasonable limits. The same system has also been identified in other animals such as Antarctic penguins, where it occurs in their legs and wings, and dolphins, which use a heat exchange system in their fins.