Editing Negative feedback (section)

== Areas of application ==

=== Mechanical engineering ===
{{See also|Control systems|Control engineering}}
[[File:Ballcock.svg|thumb|The [[ballcock]] or float valve uses negative feedback to control the water level in a cistern.]]
An example of the use of negative feedback control is the [[ballcock]] control of water level (see diagram), or a [[pressure regulator]]. In modern engineering, negative feedback loops are found in [[Governor (device)|engine governor]]s, [[fuel injection]] systems and [[carburettor]]s. Similar control mechanisms are used in heating and cooling systems, such as those involving [[air conditioners]], [[refrigerators]], or [[freezers]].

=== Biology ===
{{See also|Counterregulatory hormone|Homeostasis}}
[[Image:ACTH Negative Feedback.svg|thumb|250px|Control of endocrine hormones by negative feedback.]]

Some biological systems exhibit negative feedback such as the [[baroreflex]] in [[blood pressure]] regulation and [[erythropoiesis]]. Many biological processes (e.g., in the [[human anatomy]]) use negative feedback. Examples of this are numerous, from the regulating of body temperature, to the regulating of blood [[glucose]] levels. The disruption of feedback loops can lead to undesirable results: in the case of [[Blood sugar level|blood glucose levels]], if negative feedback fails, the glucose levels in the blood may begin to rise dramatically, thus resulting in [[Diabetes mellitus|diabetes]].

For hormone secretion regulated by the negative feedback loop: when gland X releases hormone X, this stimulates target cells to release hormone Y. When there is an excess of hormone Y, gland X "senses" this and inhibits its release of hormone X. As shown in the figure, most [[endocrine]] [[hormone]]s are controlled by a [[physiology|physiologic]] negative feedback inhibition loop, such as the [[glucocorticoid]]s secreted by the [[adrenal cortex]].  The [[hypothalamus]] secretes [[Corticotropin-releasing hormone|corticotropin-releasing hormone (CRH)]], which directs the [[pituitary gland|anterior pituitary gland]] to secrete [[ACTH|adrenocorticotropic hormone (ACTH)]].  In turn, ACTH directs the adrenal cortex to secrete glucocorticoids, such as [[cortisol]].  Glucocorticoids not only perform their respective functions throughout the body but also negatively affect the release of further stimulating secretions of both the hypothalamus and the pituitary gland, effectively reducing the output of glucocorticoids once a sufficient amount has been released.<ref>Raven, PH; Johnson, GB. ''Biology'', Fifth Edition, Boston: Hill Companies, Inc. 1999. page 1058.</ref>

=== Chemistry ===

Closed systems containing substances undergoing a [[reversible reaction|reversible chemical reaction]] can also exhibit negative feedback in accordance with [[Le Chatelier's principle]] which shift the [[chemical equilibrium]] to the opposite side of the reaction in order to reduce a stress. For example, in the reaction 
: N<sub>2</sub> + 3 H<sub>2</sub> ⇌ 2 NH<sub>3</sub> + 92 kJ/mol
If a mixture of the reactants and products exists at equilibrium in a sealed container and nitrogen gas is added to this system, then the equilibrium will shift toward the product side in response. If the temperature is raised, then the equilibrium will shift toward the reactant side which, since the reverse reaction is endothermic, will partially reduce the temperature.

===Self-organization===
{{main|Self-organization|Emergence}}
Self-organization is the capability of certain systems "of organizing their own behavior or structure".<ref name=Uttal>
{{cite book |url=https://books.google.com/books?id=vil6AgAAQBAJ&pg=PA95 |author= William R. Uttal |title=Psychomythics: Sources of Artifacts and Misconceptions in Scientific Psychology |isbn=9781135623722 |publisher=Psychology Press |year=2014 |pages=95 ''ff''}}
</ref> There are many possible factors contributing to this capacity, and most often [[positive feedback]] is identified as a possible contributor. However, negative feedback also can play a role.<ref name=Camazine>
{{cite book |title=Self-organization in biological systems |author1=Scott Camazine |author2=Jean-Louis Deneubourg |author3=Nigel R Franks |author4=James Sneyd |author5=Guy Theraulaz |author6=Eric Bonabeau |chapter=Chapter 2: How self-organization works |pages=15 ''ff'' |isbn= 9780691116242 |year=2003 |publisher=Princeton University Press |chapter-url=https://books.google.com/books?id=zMgyNN6Ufj0C&pg=PA15}}
</ref>

=== Economics ===

In economics, [[automatic stabiliser]]s are government programs that are intended to work as negative feedback to dampen fluctuations in [[real GDP]].

[[Mainstream economics]] asserts that the market pricing mechanism operates to match [[supply and demand]], because mismatches between them feed back into the decision-making of suppliers and demanders of goods, altering prices and thereby reducing any discrepancy. However [[Norbert Wiener]] wrote in 1948:

:''"There is a belief current in many countries and elevated to the rank of an official article of faith in the United States that free competition is itself a homeostatic process... Unfortunately the evidence, such as it is, is against this simple-minded theory."''<ref>[[Cybernetics: Or Control and Communication in the Animal and the Machine]] p.158</ref>

The notion of economic equilibrium being maintained in this fashion by market forces has also been questioned by numerous [[heterodox economics|heterodox]] economists such as [[financier]] [[George Soros]]<ref>Goeroge Soros, ''[https://books.google.com/books?id=qxkiYul2wgoC The Alchemy of Finance]''</ref> and leading [[Ecological economics|ecological economist]] and [[Steady-state economy#Herman Daly's concept of a steady-state economy|steady-state theorist]]  [[Herman Daly]], who was with the [[World Bank]] in 1988–1994.<ref>Herman Daly, ''[https://books.google.com/books?id=DwC8BwAAQBAJ Steady State Economics]''</ref>

=== Environmental Science ===
[[File:20220726 Feedbacks affecting global warming and climate change - block diagram.svg |thumb|right|upright=1.5| Some [[effects of climate change]] can either enhance ([[positive feedback]]s) or weaken (negative feedbacks) global warming.<ref name=NASA_IntegratedSystem>{{cite web |title=The Study of Earth as an Integrated System |url=https://climate.nasa.gov/nasa_science/science/ |website=nasa.gov |publisher=NASA |date=2016 |archive-url=https://web.archive.org/web/20161102022200/https://climate.nasa.gov/nasa_science/science/ |archive-date=November 2, 2016 |url-status=live }}</ref><ref name=IPCC_AR6_SGI_FigTS.17>Fig. TS.17, ''[https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_TS.pdf Technical Summary],'' Sixth Assessment Report (AR6), Working Group I, IPCC, 2021, p. 96. [https://web.archive.org/web/20220721021347/https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_TS.pdf Archived] from the original on July 21, 2022.</ref>]]
A basic and common example of a negative feedback system in the environment is the interaction among [[cloud cover]], plant growth, [[Solar irradiance|solar radiation]], and planet temperature.<ref>{{Cite journal|last1=Charlson|first1=Robert J.|last2=Lovelock|first2=James E.|last3=Andreae|first3=Meinrat O.|last4=Warren|first4=Stephen G.|date=1987|title=Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate|journal=Nature|language=en|volume=326|issue=6114|pages=655–661|doi=10.1038/326655a0|issn=1476-4687|bibcode=1987Natur.326..655C|s2cid=4321239}}</ref> As incoming solar radiation increases, planet temperature increases. As the temperature increases, the amount of plant life that can grow increases. This plant life can then make products such as sulfur which produce more cloud cover. An increase in cloud cover leads to higher [[albedo]], or surface reflectivity, of the Earth. As albedo increases, however, the amount of solar radiation decreases.<ref>{{Cite journal|last=Winton|first=Michael|date=2006|title=Amplified Arctic climate change: What does surface albedo feedback have to do with it?|journal=Geophysical Research Letters|language=en|volume=33|issue=3|pages=L03701|doi=10.1029/2005GL025244|issn=1944-8007|bibcode=2006GeoRL..33.3701W|doi-access=free}}</ref> This, in turn, affects the rest of the cycle.

Cloud cover, and in turn planet albedo and temperature, is also influenced by the [[Water cycle|hydrological cycle]].<ref>{{Cite journal|last=Stephens|first=Graeme L.|date=2005|title=Cloud Feedbacks in the Climate System: A Critical Review|journal=Journal of Climate|volume=18|issue=2|pages=237–273|doi=10.1175/JCLI-3243.1|issn=0894-8755|bibcode=2005JCli...18..237S|s2cid=16122908 |doi-access=free}}</ref> As planet temperature increases, more water vapor is produced, creating more clouds.<ref>{{Cite journal|last1=Jickells|first1=T. D.|last2=An|first2=Z. S.|last3=Andersen|first3=K. K.|last4=Baker|first4=A. R.|last5=Bergametti|first5=G.|last6=Brooks|first6=N.|last7=Cao|first7=J. J.|last8=Boyd|first8=P. W.|last9=Duce|first9=R. A.|last10=Hunter|first10=K. A.|last11=Kawahata|first11=H.|date=2005|title=Global Iron Connections Between Desert Dust, Ocean Biogeochemistry, and Climate|journal=Science|language=en|volume=308|issue=5718|pages=67–71|doi=10.1126/science.1105959|issn=0036-8075|pmid=15802595|bibcode=2005Sci...308...67J|s2cid=16985005}}</ref> The clouds then block incoming solar radiation, lowering the temperature of the planet. This interaction produces less [[water vapor]] and therefore less cloud cover. The cycle then repeats in a negative feedback loop. In this way, negative feedback loops in the environment have a stabilizing effect.<ref>{{Cite journal|last1=Giannini|first1=Alessandra|last2=Biasutti|first2=Michela|last3=Verstraete|first3=Michel M.|date=2008|title=A climate model-based review of drought in the Sahel: Desertification, the re-greening and climate change|journal=Global and Planetary Change|series=Climate Change and Desertification|volume=64|issue=3|pages=119–128|doi=10.1016/j.gloplacha.2008.05.004|issn=0921-8181|bibcode=2008GPC....64..119G}}</ref>