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.

K. Schmidt-Nielsen, “Countercurrent Systems in Animals,” Scientific American, Vol. 244, 1981, pp. 118-128. ... The peri-arterial venous network in the paw pad formed a countercurrent heat exchanger in ...

The arteriovenous diffusion of heat explains the higher temperatures observed in the cortex and the renal medulla than in renal venous blood. The arteriovenous thermodilution technique is unsuited for ...

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In these animals, the heat exchanger cools the blood flowing to the brain to reduce overheating (Tattersall et al., 2006). This hypothesis is of course quite speculative, ... Vascular anatomy of the ...