Countercurrent Heat Exchange in Arctic Legs

Arctic foxes, reindeer, and wading birds keep their core warm while standing on ice by running the artery and vein of each limb side by side: heat leaks continuously from the outbound warm arterial blood into the inbound cool venous blood along the whole length of the limb, so blood re-entering the body has already been pre-warmed. This simulation solves that real countercurrent heat-exchanger balance segment by segment along a 3D limb model, driven by blood flow rate, vessel contact efficiency, ambient cold, and insulation, and lets you toggle the exchange off to see just how much more heat a limb would lose without it. Click either vessel to read its local temperature.