Editing Homeostasis (section)

===Core temperature===
{{Main |Thermoregulation |Thermoregulation in humans}}
{{Further |Preoptic area}}
[[File: Smallhuddle.jpg|thumb|Birds huddling for warmth]]
[[Mammal]]s regulate their [[Human body temperature#Core temperature|core temperature]] using input from [[thermoreceptor]]s in the [[hypothalamus]], brain,<ref name=tortora2 /><ref name=grays>{{cite book |last1=Williams |first1=Peter L. |last2=Warwick |first2=Roger |last3=Dyson|first3=Mary |last4=Bannister |first4=Lawrence H. |title=Gray's Anatomy| pages=691–692, 791, 10011–10012 |location=Edinburgh |publisher=Churchill Livingstone | edition= Twenty-seventh
|date=1989|isbn= 0-443-04177-6 }}</ref> [[spinal cord]], [[organs#(anatomy)|internal organs]], and great veins.<ref>{{cite journal|last1=Tansey|first1=Etain A.|last2=Johnson|first2=Christopher D|date=2015|title=Recent advances in thermoregulation|journal=Advances in Physiology Education|volume=39|issue=3|pages=139–148|doi=10.1152/advan.00126.2014|issn=1043-4046|pmid=26330029|s2cid=11553866 |url=https://pure.qub.ac.uk/portal/en/publications/recent-advances-in-thermoregulation-review(c59df53e-4325-4c81-b52b-4e337891c0bb).html|access-date=26 January 2019|archive-date=10 May 2020|archive-url=https://web.archive.org/web/20200510225628/https://pure.qub.ac.uk/en/publications/recent-advances-in-thermoregulation-review|url-status=live}}</ref><ref name=":0">{{Cite book|title=Gray's anatomy : the anatomical basis of clinical practice|others=Standring, Susan|isbn=978-0-7020-6851-5|edition= 41st|location=[Philadelphia]|pages=141, 151–152|oclc=920806541|last1 = Standring|first1 = Susan|date=7 August 2015}}</ref> Apart from the internal regulation of temperature, a process called [[allostasis]] can come into play that adjusts behaviour to adapt to the challenge of very hot or cold extremes (and to other challenges).<ref name="Purves">{{cite book|last1=Purves|first1=Dale|title=Neuroscience|date=2011|publisher=Sinauer|location=Sunderland, Mass.|isbn=978-0-87893-695-3|page=458|edition= 5th}}</ref> These adjustments may include seeking shade and reducing activity, seeking warmer conditions and increasing activity, or huddling.<ref name= biology>{{cite book|last1=Campbell |first1=Neil A. |title=Biology |edition=  Second |pages=897–898 |location=Redwood City, California  |publisher=The Benjamin/Cummings Publishing Company |date=1990 |isbn= 978-0-8053-1800-5}}</ref>
Behavioral thermoregulation takes precedence over physiological thermoregulation since necessary changes can be affected more quickly and physiological thermoregulation is limited in its capacity to respond to extreme temperatures.<ref name="Flouris">{{cite journal|last1=Flouris|first1=AD|title=Functional architecture of behavioural thermoregulation.|journal=European Journal of Applied Physiology|date=January 2011|volume=111|issue=1|pages=1–8|doi=10.1007/s00421-010-1602-8|pmid=20711785|s2cid=9109352}}</ref>

When the core temperature falls, the blood supply to the skin is reduced by intense [[vasoconstriction]].<ref name=tortora2 /> The blood flow to the limbs (which have a large surface area) is similarly reduced and returned to the trunk via the deep veins which lie alongside the arteries (forming [[vena comitans|venae comitantes]]).<ref name=grays /><ref name= biology /><ref>{{cite book |last1=Gilroy |first1=Anne M. |last2=MacPherson |first2=Brian R. |last3=Ross|first3=Lawrence M. |title=Atlas of Anatomy| pages=318, 349  |location=Stuttgart|publisher=Thieme Medical Publishers|date=2008|isbn= 978-1-60406-062-1 }}</ref> This acts as a [[Countercurrent exchange|counter-current exchange system]] that short-circuits the warmth from the arterial blood directly into the venous blood returning into the trunk, causing minimal heat loss from the extremities in cold weather.<ref name=grays /><ref name= biology /><ref name="pmid7233149">{{cite journal |vauthors = Schmidt-Nielsen K |title = Countercurrent systems in animals |journal = Scientific American |volume = 244 |issue = 5 |pages = 118–28 |year = 1981 |pmid = 7233149 |doi=10.1038/scientificamerican0581-118|bibcode = 1981SciAm.244e.118S }}</ref> The subcutaneous limb veins are tightly constricted,<ref name=tortora2 /> not only reducing heat loss from this source but also forcing the venous blood into the counter-current system in the depths of the limbs.

The metabolic rate is increased, initially by non-shivering [[thermogenesis]],<ref>{{cite book |last1=Stuart |first1=I.R. |title=Human physiology.| page=667|location=New York|publisher=McGraw-Hill | edition= Twelfth |date=2011 }}</ref> followed by [[Shivering|shivering thermogenesis]] if the earlier reactions are insufficient to correct the [[hypothermia]].

When core temperature rises are detected by [[thermoreceptor]]s, the [[sweat gland]]s in the skin are stimulated via [[cholinergic]] [[Sympathetic nervous system|sympathetic nerves]] to secrete [[Sweat gland#Sweat|sweat]] onto the skin, which, when it evaporates, cools the skin and the blood flowing through it. Panting is an alternative effector in many vertebrates, which cools the body also by the evaporation of water, but this time from the [[mucous membranes]] of the throat and mouth.<ref>{{Cite journal |last=Robertshaw |first=David |date=August 2006 |title=Mechanisms for the control of respiratory evaporative heat loss in panting animals |url=https://pubmed.ncbi.nlm.nih.gov/16675613 |journal=Journal of Applied Physiology |volume=101 |issue=2 |pages=664–668 |doi=10.1152/japplphysiol.01380.2005 |issn=8750-7587 |pmid=16675613}}</ref>