Countercurrent Heat Exchange in Animal Limbs

Many cold-adapted and aquatic animals keep their core warm without wasting energy heating an entire limb: arteries and veins run in tight parallel bundles so outgoing warm blood pre-heats the cooled blood returning from the extremity, and vice versa. This simulator solves a discretized 1D countercurrent heat-exchanger model — advection along the artery and vein plus a direct thermal coupling between them and heat loss to the environment at the extremity — and renders the limb in 3D as two color-coded vessels with animated flow. Adjust ambient temperature, core body temperature, blood flow rate and coupling strength, or flip the vein to co-current flow, and watch the extremity temperature and the fraction of core heat retained respond live.