Editing Heat exchanger (section)

==In nature==

===Humans===
The human nasal passages serve as a heat exchanger, with cool air being inhaled and warm air being exhaled.  Its effectiveness can be demonstrated by putting the hand in front of the face and exhaling, first through the nose and then through the mouth.  Air exhaled through the nose is substantially cooler.<ref>{{cite web |url=https://apps.dtic.mil/sti/citations/AD0293194 |title=Heat Loss from the Respiratory Tract in Cold |publisher=Defense Technical Information Center |date=April 1955 |last1=Webb|first1=Paul |url-status=dead |archive-url= https://web.archive.org/web/20220808095847/https://apps.dtic.mil/sti/citations/AD0293194 |archive-date= Aug 8, 2022 }}</ref><ref>{{cite book |title=Eckert animal physiology: mechanisms and adaptations |last=Randall |first=David J. |author2=Warren W. Burggren |author3=Kathleen French |author4=Roger Eckert |year=2002 |publisher=Macmillan |isbn=978-0-7167-3863-3 |page=[https://archive.org/details/eckertanimalphys00rand/page/587 587] |url-access=registration |url=https://archive.org/details/eckertanimalphys00rand/page/587 }}</ref> This effect can be enhanced with clothing, by, for example, wearing a scarf over the face while breathing in cold weather.

In species that have external testes (such as human), the artery to the testis is surrounded by a mesh of veins called the [[pampiniform plexus]]. This cools the blood heading to the testes, while reheating the returning blood.

===Birds, fish, marine mammals===
[[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 circulatory 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 tongues of [[baleen whale]]s as large volumes 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===
Carotid rete is a counter-current heat exchanging organ in some [[ungulate]]s. The blood ascending the [[carotid arteries]] on its way to the brain, flows via a network of vessels where heat is discharged to the veins of cooler blood descending from the nasal passages. The carotid rete allows [[Thomson's gazelle]] to maintain its brain almost 3&nbsp;°C (5.4&nbsp;°F) cooler than the rest of the body, and therefore aids in tolerating bursts in metabolic heat production such as associated with outrunning [[cheetah]]s (during which the body temperature exceeds the maximum temperature at which the brain could function).<ref>{{cite web|url=https://asknature.org/strategy/carotid-rete-cools-brain/|title=Carotid rete cools brain : Thomson's Gazelle}}</ref> Humans with other primates lack a carotid rete.<ref name="Bruner Mantini Musso De La Cuétara 2010 pp. 35–43">{{cite journal |last1=Bruner |first1=Emiliano |last2=Mantini |first2=Simone |last3=Musso |first3=Fabio |last4=De La Cuétara |first4=José Manuel |last5=Ripani |first5=Maurizio |last6=Sherkat |first6=Shahram |date=2010-11-30 |title=The evolution of the meningeal vascular system in the human genus: From brain shape to thermoregulation |journal=American Journal of Human Biology |volume=23 |issue=1 |pages=35–43 |doi=10.1002/ajhb.21123 |pmid=21120884 |issn=1042-0533}}</ref>