BeCuriouzLearn something new every day
Why a duck can stand on ice and never feel the cold in its feet
💡 By running warm outgoing blood next to cold returning blood, countercurrent exchange recycles heat so animals keep their extremities cold-tolerant without losing precious body warmth.
A duck paddling in near-freezing water should be losing enormous amounts of body heat through its bare feet. It doesn't — thanks to a plumbing trick called countercurrent heat exchange. In the duck's legs, the arteries carrying warm blood down to the feet run right alongside the veins carrying cold blood back up. The two flow in opposite directions, pressed close together.
Because they're side by side, heat leaks sideways from the warm outgoing blood into the cold returning blood. Warmth gets recycled back into the body before it ever reaches the feet, and the cold blood is pre-warmed before it hits the core. The result: the feet are kept just above freezing — cold enough to lose almost no heat to the ice, but never frozen.
The same principle keeps a penguin's feet, a tuna's swimming muscles, and a wading heron's legs from bleeding heat into their surroundings. Engineers borrowed the exact idea for heat-recovery ventilators, which use outgoing stale air to pre-warm incoming fresh air. Nature and HVAC systems solved the same problem the same way.