Countercurrent heat exchange

Ever wonder how dogs keep their tootsies toasty in the freezing cold?


Every winter it takes me way longer than it should to dig out my hat and gloves from the closet. Last week’s near record-breaking cold put an end to my denial, and I now leave the house well-swaddled in muffler, hat, and mittens. Yet my dog and cats still are romping outside, seemingly unfazed by the bitter weather. How do they do that, bare-pawed in the snow? Before going further, let me be clear. Dogs and cats get cold, and just like us, are susceptible to frostbite.

Let’s start by defining our terms. The first stage of frostbite is poetically called “frostnip.” In people, the skin gets either pale or red and is very cold. You may feel prickling, numbness, pain, or tingling, but it won’t cause permanent skin damage, and the only treatment needed is gentle rewarming. True frostbite is the result of freezing of the skin and underlying tissues. It can result in loss of the affected tissues, secondary infection, even death. In people it is most common on the fingers, toes, nose, ears, cheeks, and chin. In dogs and cats, it’s the ears, nose, and feet. Severe frostbite requires prompt professional attention.  

An ounce of prevention is worth a pound of cure. Use common sense. Keep pets inside during these cold snaps as much as possible. For the most fragile individuals — the elderly, small, thin, or ill — limit exposure to these frigid temperatures to just the briefest time needed for them to relieve themselves, and it’s a good idea to dress them in sweaters, coats, even booties … which brings me to today’s topic: Cold feet. Exactly how does it work that Hardy the husky doesn’t routinely get frostbite on his paws as he cavorts through the Arctic cold?

In a 2011 article in Veterinary Dermatology titled “Functional anatomy of footpad vasculature in dogs: Scanning electron microscopy of vascular corrosion casts,” researchers looked at the mechanisms that keep dogs’ paws from freezing. The authors note, “Dogs are well adapted to cold climates, and they can stand, walk, and run on snow and ice for long periods of time. In contrast to the body trunk, which has dense fur, the paws are more exposed to the cold due to the lack of fur insulation. The extremities have a high surface area-to-volume ratio, so they lose heat very easily.” OK, that seems pretty obvious. So what’s the trick to keeping those tootsies toasty?

The researchers made detailed molds of the veins and arteries in canine footpads. They also examined histological specimens in which these vessels were stained with India ink. What they found were complex patterns of blood flow that were adaptive to cold weather in two ways. The first adaptation noted was that each footpad artery was sandwiched between two veins. In this way, heat in the feet was conserved. Got it? The warm blood coming into the feet via arteries was insulated by a surrounding network of veins flowing in the opposite direction. The heat from the blood going in one direction warms the blood going in the other direction, preventing heat loss by recirculating it back to the body core. It’s called countercurrent heat exchange.

A second vascular mechanism, called arteriovenous anastomoses, was also identified. This allows blood flow to shift when a dog is in cold environments, draining blood to the skin surface, but retaining warm blood in the pad surface. Other studies have identified similar systems in the feet of Arctic foxes and gray wolves. These animals appear to be able to regulate blood flow and heat exchange in their feet, keeping their paws at just one degree above the temperature at which tissue damage would occur.

Countercurrent heat exchange can also work in the opposite direction, in other words, to eliminate excessive heat. Several types of large antelopes, such as the oryx, gemsbok, and eland, live on the African plains, where environmental temperatures rise so high they pose a risk of causing brain damage. How do these animals survive on the sweltering, sunbeaten plains? Countercurrent heat exchange! Blood going from the heart toward the brain travels via the carotid arteries. In these animals, the carotid arteries break up into many much smaller vessels called the rete mirabile, which then pass through a venous sinus. The blood in the venous sinus has previously circulated along the nasal passages, where evaporative cooling from the moist membranes that line the nose has dropped the temperature of the blood. The cool venous blood then cools the arterial blood headed for the brain, dropping the temperature as much as three degrees, protecting the oryx from neurological damage and heat stroke.

The oryx is particularly near and dear to my heart. I went to a somewhat alternative college for my undergraduate degree, where we were encouraged to follow our interests wherever they might lead. I had been particularly fascinated by environmental adaptations in zoology, and had learned quite a bit about the subject. In my senior year, while applying to veterinary school, I was invited to Cornell for an interview, during which the interviewer asked me about some of my studies. I went off on a bit of a rant about the beauty and awe of how Mother Nature crafts these ingenious mechanisms for survival. “Like the oryx!” I concluded triumphantly. The interviewers looked at me blankly. “You know,” I persisted, “the oryx? Countercurrent heat exchange? Cooling the brain?”

I was put on the waitlist at Cornell. I guess those particular faculty were not fans of the oryx. (When I later went to my interview at the University of Pennsylvania, I decided not to mention the oryx, and I was accepted. Hmmm.) But I still marvel at these adaptations, the elegance with which Mother Nature solves problems, keeping an oryx’s brain cool and a dog’s paws warm. Amazing.