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[[File:Countercurrent conservator.svg|thumb|right|upright=1.35|Counter-current exchange conservation circuit]]
{{further|Countercurrent exchange#Counter-current exchange in biological systems{{!}}Counter-current exchange in biological systems }}
"[[Countercurrent exchange|Countercurrent]]" heat exchangers occur naturally in the circulationcirculatory systems of [[fish]], [[whale]]s and other [[marine mammals]]. Arteries to the skin carrying warm blood are intertwined with veins from the skin carrying cold blood, causing the warm arterial blood to exchange heat with the cold venous blood. This reduces the overall heat loss in cold water. Heat exchangers are also present in the tongue of [[baleen whale]]s as large volume of water flow through their mouths.<ref>{{cite web|url=http://www.nhm.org/research/mammals/jj/ |archive-url=https://web.archive.org/web/20090614084555/http://www.nhm.org/research/mammals/jj/ |url-status=dead |archive-date=2009-06-14 |title=Natural History Museum: Research & Collections: History |access-date=2019-09-09}}</ref><ref>{{cite journal |doi=10.1126/science.278.5340.1138 |journal=Science |volume=278 |issue=5340 |author=Heyning and Mead |pages=1138–1140 |date=November 1997 |title=Thermoregulation in the Mouths of Feeding Gray Whales |pmid=9353198 |last2=Mead |first2=JG|bibcode=1997Sci...278.1138H }}</ref> Wading birds use a similar system to limit heat losses from their body through their legs into the water.
===Carotid rete===
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