Editing Reindeer (section)

=== Heat exchange ===
Blood moving into the legs is cooled by blood returning to the body in a [[Countercurrent exchange|countercurrent heat exchange]] (CCHE), a highly efficient means of minimizing heat loss through the skin's surface. In the CCHE mechanism, in cold weather, blood vessels are closely knotted and intertwined with arteries to the skin and appendages that carry warm blood with veins returning to the body that carry cold blood causing the warm arterial blood to exchange heat with the cold venous blood. In this way, their legs for example are kept cool, maintaining the core body temperature nearly {{cvt|30|C-change}} higher with less heat lost to the environment. Heat is thus recycled instead of being dissipated. The "heart does not have to pump blood as rapidly in order to maintain a constant body core temperature and thus, metabolic rate." CCHE is present in animals like reindeer, fox and moose living in extreme conditions of cold or hot weather as a mechanism for retaining the heat in (or out of) the body. These are countercurrent exchange systems with the same fluid, usually blood, in a circuit, used for both directions of flow.<ref name="Rahiman2009">{{cite web |first=Mohd Hezri Fazalul |last=Rahiman |url=http://hezrif.uitm.edu.my/ese652/51%20Heat%20Exchanger%20090907d.pdf |title=Heat exchanger |year=2009 |access-date=3 November 2014 |location=Malaysia |archive-url=https://web.archive.org/web/20131205011425/http://hezrif.uitm.edu.my/ese652/51%20Heat%20Exchanger%20090907d.pdf |archive-date=5 December 2013 }}</ref>

Reindeer have specialized counter-current vascular heat exchange in their [[Nasal cavity|nasal passage]]s. Temperature gradient along the nasal [[Mucous membrane|mucosa]] is under physiological control. Incoming cold air is warmed by [[Thermoregulation|body heat]] before entering the lungs and water is condensed from the expired air and captured before the reindeer's breath is exhaled, then used to moisten dry incoming air and possibly be absorbed into the blood through the [[mucous membrane]]s.<ref name="BlixJohnsen1983">{{cite journal |last1=Blix |first1=A.S. |last2=Johnsen |first2=Helge Kreiitzer|pmid=6887057|pmc=1199219 |title=Aspects of nasal heat exchange in resting reindeer |journal=Journal of Physiology |volume=340 |year=1983 |pages=445–454 |doi=10.1113/jphysiol.1983.sp014772 }}</ref> Like [[moose]], caribou have specialized noses featuring nasal [[Nasal concha|turbinate bone]]s that dramatically increase the surface area within the [[nostril]]s.