Editing Homeostasis (section)

==Overview==

The [[Metabolism|metabolic processes]] of all organisms can only take place in very specific physical and chemical environments. The conditions vary with each organism, and also with whether the chemical processes take place inside the [[Cell (biology)|cell]] or in the [[interstitial fluid]] bathing the cells. The best-known homeostatic mechanisms in humans and other mammals are regulators that keep the composition of the [[extracellular fluid]] (or the "internal environment") constant, especially with regard to the [[Thermoregulation|temperature]], [[pH]], [[osmolality]], and the concentrations of [[sodium]], [[potassium]], [[glucose]], [[carbon dioxide]], and [[oxygen]]. However, a great many other homeostatic mechanisms, encompassing many aspects of [[human physiology]], control other entities in the body. Where the levels of variables are higher or lower than those needed, they are often prefixed with ''hyper-'' and ''hypo-'', respectively such as [[hyperthermia]] and [[hypothermia]] or [[hypertension]] and [[hypotension]].{{citation needed|date=April 2023}}

[[Image:Body Temp Variation.svg|class=skin-invert-image|thumb|250px|right|Circadian variation in body temperature, ranging from about 37.5&nbsp;°C from 10 a.m. to 6 p.m., and falling to about 36.4&nbsp;°C from 2 a.m. to 6 a.m.]]
If an entity is homeostatically controlled it does not imply that its value is necessarily absolutely steady in health. [[Thermoregulation|Core body temperature]] is, for instance, regulated by a homeostatic mechanism with temperature sensors in, amongst others, the [[hypothalamus]] of the [[brain]].<ref name=tortora2>{{cite book |last1=Tortora |first1=Gerard J. | last2=Anagnostakos |first2=Nicholas P. | title=Principles of Anatomy and Physiology |url=https://archive.org/details/principlesofan1987tort |url-access=registration | edition= Fifth |location=New York |publisher=Harper & Row, Publishers |date=1987 |pages=[https://archive.org/details/principlesofan1987tort/page/315 315]–316, 475, 657–658 |isbn=978-0-06-350729-6 }}</ref> However, the [[Setpoint (control system)|set point]] of the regulator is regularly reset.<ref>{{cite web |last1=Khan Academy |title=Homeostasis |url=https://www.khanacademy.org/science/high-school-biology/hs-human-body-systems/hs-body-structure-and-homeostasis/a/homeostasis |website=Khan Academy |access-date=13 July 2018 |archive-date=20 October 2019 |archive-url=https://web.archive.org/web/20191020034655/https://www.khanacademy.org/science/high-school-biology/hs-human-body-systems/hs-body-structure-and-homeostasis/a/homeostasis |url-status=live }}</ref> For instance, core body temperature in humans [[Thermoregulation#Variations due to circadian rhythms|varies during the course of the day]] (i.e. has a [[circadian rhythm]]), with the lowest temperatures occurring at night, and the highest in the afternoons. Other normal [[Human body temperature|temperature variations]] include those related to the [[menstrual cycle]].<ref>{{cite book |url=https://books.google.com/books?id=JZb0ibgYDCIC&pg=PA149 |title=Women's Sports Medicine and Rehabilitation |isbn=978-0-8342-1731-7 |publisher=Lippincott Williams & Wilkins |year=2001 |last=Swedan |first=Nadya Gabriele |pages=149 |access-date=11 October 2019 |archive-date=10 May 2020 |archive-url=https://web.archive.org/web/20200510140218/https://books.google.com/books?id=JZb0ibgYDCIC&pg=PA149 |url-status=live }}</ref><ref name=tcoyf2>{{cite book | first=Toni | last=Weschler | year=2002 | title=Taking Charge of Your Fertility | url=https://archive.org/details/takingchargeofyo00toni | url-access=registration | pages=[https://archive.org/details/takingchargeofyo00toni/page/52 52], 316, 361–362 | publisher=HarperCollins | location=New York | isbn=978-0-06-093764-5 }}</ref> The temperature regulator's set point is reset during infections to produce a fever.<ref name=tortora2 /><ref>{{cite book|last=Kluge |first=Matthew J. |title=Fever: Its Biology, Evolution, and Function|date=2015|publisher=Princeton University Press |isbn=978-1-4008-6983-1|page=57 |url=https://books.google.com/books?id=gIF9BgAAQBAJ&pg=PA57}}</ref><ref name=Gar2012p375>{{cite book|last1=Garmel|first1=Gus M.|editor-last1=Mahadevan|editor-first1=S.V.|editor-last2=Garmel|editor-first2=Gus M.|title=An introduction to clinical emergency medicine|chapter=Fever in adults|date=2012|publisher=Cambridge University Press|location=Cambridge|isbn=978-0-521-74776-9|pages=375|edition=2nd|chapter-url=https://books.google.com/books?id=pyAlcOfBhjIC&pg=PA375|access-date=11 October 2019|archive-date=30 December 2019|archive-url=https://web.archive.org/web/20191230070045/https://books.google.com/books?id=pyAlcOfBhjIC&pg=PA375|url-status=live}}</ref> Organisms are capable of adjusting somewhat to varied conditions such as temperature changes or oxygen levels at altitude, by a process of [[acclimatization|acclimatisation]].

Homeostasis does not govern every activity in the body.<ref>{{Cite book|title = Where Medicine Went Wrong: Rediscovering the Path to Complexity|publisher = World Scientific|location = New Jersey|series = Studies of Nonlinear Phenomena in Life Science|volume = 11|last = West|first = Bruce J|doi = 10.1142/6175|isbn = 978-981-256-883-0|url = https://books.google.com/books?id=qdZoDQAAQBAJ|date = 2006|access-date = 23 January 2019|archive-date = 6 March 2022|archive-url = https://web.archive.org/web/20220306112837/https://books.google.com/books?id=qdZoDQAAQBAJ|url-status = live}}</ref><ref>{{Cite book|title=Perspectives on Organisms |publisher= Springer |last1=Longo |first1=Giuseppe |last2=Montévil |first2=Maël |doi=10.1007/978-3-642-35938-5|series = Lecture Notes in Morphogenesis|year = 2014|isbn = 978-3-642-35937-8|s2cid= 27653540 }}</ref> For instance, the signal (be it via [[neuron]]s or [[hormone]]s) from the sensor to the effector is, of necessity, highly variable in order to convey [[Information theory|information]] about the direction and magnitude of the error detected by the sensor.<ref name="Shannon">{{cite book|last1= Shannon |first1=Claude E.|last2=Weaver|first2=Warren|title=The mathematical theory of communication|date=1963|publisher=University of Illinois Press|location=Urbana|isbn=978-0-252-72548-7|edition= 4. print.}}</ref><ref>{{cite book |last1=Rucker |first1=R. |title=Mind tools: the mathematics of information.|pages=25–30 |location=Harmondsworth|publisher=Penguin Books |date=1987 }}</ref><ref>{{cite journal |last1= Koeslag |first1=Johan H. |last2=Saunders |first2=Peter T. |last3=Wessels |first3=Jabus A. | title=The chromogranins and counter-regulatory hormones: do they make homeostatic sense? |journal= Journal of Physiology | publication-date=1999 |volume=517 |issue=3 |pages=643–649 |doi= 10.1111/j.1469-7793.1999.0643s.x|pmid=10358106 |year=1999 |pmc=2269385 }}</ref>  Similarly, the effector's response needs to be highly adjustable to reverse the error – in fact it should be very nearly in proportion (but in the opposite direction) to the error that is threatening the internal environment.<ref name="Hall" /><ref name=milsum /> For instance, [[arterial blood pressure]] in mammals is homeostatically controlled and measured by [[stretch receptor]]s in the walls of the [[aortic arch]] and [[carotid sinus]]es at the beginnings of the [[internal carotid arteries]].<ref name=tortora2 /> The sensors send messages via [[sensory nerve]]s to the [[medulla oblongata]] of the brain indicating whether the [[blood pressure]] has fallen or risen, and by how much. The medulla oblongata then distributes messages along [[Motor neuron|motor or efferent nerves]] belonging to the [[autonomic nervous system]] to a wide variety of effector organs, whose activity is consequently changed to reverse the error in the blood pressure. One of the effector organs is the heart whose rate is stimulated to rise ([[tachycardia]]) when the arterial blood pressure falls, or to slow down ([[bradycardia]]) when the pressure rises above the set point.<ref name=tortora2 /> Thus the heart rate (for which there is no sensor in the body) is not homeostatically controlled but is one of the effector responses to errors in arterial blood pressure. Another example is the rate of [[sweating]]. This is one of the effectors in the homeostatic control of body temperature, and therefore highly variable in rough proportion to the heat load that threatens to destabilize the body's core temperature, for which there is a sensor in the [[hypothalamus]] of the brain.{{citation needed|date=April 2023}}