Written by Peter Pearsall/Photo by Flickr user sankax (CC BY-NC 2.0)
Birds, like all mammals and some fish, are homeotherms, meaning that they internally regulate their body temperature by burning calories to create and conserve heat. The challenge of maintaining a narrow, near-constant range of internal temperature during colder months is met by various means: some animals layer on heaps of energy-rich fat to metabolize in leaner times, others rely on thick, insulating coats or feathers; and some just eat continuously. (Humans, of course, avail themselves of each methodology.) Birds that spend their winters swimming, diving, and dabbling in cold water need to take particular care in keeping warm and dry, and thus many species sport a pillowy layer of down feathers overlaid by waterproof plumage. Their feet and beaks, bared to the elements, require warmth as surely as other parts of their bodies, but the route taken is a bit more circuitous (if you’ll pardon the anatomical pun).
In order to keep those exposed areas alive and warm, blood must flow freely to and from the extremities, nourishing tissues—and it must do so economically, with minimal loss of heat. Too much heat loss leads to hypothermia; too little blood flow invites frostbite and gangrene. The bird’s circulatory system achieves this balance by employing what’s called countercurrent, the side-by-side pairing of opposite flows—in this case, venous and arterial blood. Veins transport cool, oxygen-depleted blood back to the heart, while arteries furnish tissues with the warm, oxygenated stuff. In the beaks and feet of these cold-adapted birds, the veins and arteries are intertwined like a caduceus, so that arterial heat is transferred to venous chill and vice versa, ensuring a low, stable temperature in the extremities and minimizing the unavoidable loss of heat to the water. Of additional benefit is the fact that bird legs are mostly tendon, scales, and bone—the muscles are nestled close to the warm, feathered body and thus amply insulated. Mallards, for all their incessant dabbling on slushy ponds in the dead of winter, surrender a paltry five percent of their total body heat through their orange, webby feet.
This physiological countercurrency was first discovered by Galen, the second-century Greek physician-surgeon-philosopher who called the convoluted networks of veins and arteries retia mirabilia, Latin for “miraculous nets”. It is a feat of natural engineering, remarkable in its efficiency, and since Galen’s time the countercurrent has been emulated and expanded upon by humans in a variety of industrial applications, including petroleum refining, nuclear waste processing, and the extraction of gold from nickel-cyanide slurry.