Editing Lung (section)

== Function ==
{{Main |Respiratory system |Breathing |Gas exchange}}

===Gas exchange===
The major function of the lungs is [[gas exchange]] between the lungs and the blood.<ref>{{cite book|last1=Tortora|first1=G|last2=Anagnostakos|first2=N|title=Principles of Anatomy and Physiology|year=1987|publisher=Harper and Row|isbn=978-0-06-350729-6|page=555}}</ref> The [[pulmonary alveolus|alveolar]] and [[Pulmonary circulation|pulmonary capillary]] gases equilibrate across the thin [[blood–air barrier]].<ref name=pmid26475269/><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=1278–1282 |location=Edinburgh|publisher=Churchill Livingstone | edition=37th |publication-date=1989|isbn= 0443-041776 |year=1989 }}</ref><ref name="s-cool">{{Cite web| title= Gas Exchange in humans| url=http://www.s-cool.co.uk/a-level/biology/gas-exchange/revise-it/gas-exchange-in-humans| access-date= 19 March 2013}}</ref> This thin membrane (about 0.5 –2&nbsp;μm thick) is folded into about 300 million alveoli, providing an extremely large surface area (estimates varying between 70 and 145&nbsp;m<sup>2</sup>) for gas exchange to occur.<ref name=grays /><ref>{{cite book|last1=Tortora|first1=G|last2=Anagnostakos|first2=N|title=Principles of Anatomy and Physiology|year=1987|publisher=Harper and Row|isbn=978-0-06-350729-6|page=574}}</ref>
[[File:ribcage during inhalation.jpg|thumb|The effect of the [[Muscles of respiration|respiratory muscles]] in expanding the [[rib cage]]]]

The lungs are not capable of expanding to [[breathing|breathe]] on their own, and will only do so when there is an increase in the volume of the thoracic cavity.<ref name="Levitzky2013_1">{{cite book|last1=Levitzky|first1=Michael G.|title=Pulmonary physiology|date=2013|publisher=McGraw-Hill Medical|location=New York|isbn=978-0-07-179313-1|chapter=Chapter 1. Function and Structure of the Respiratory System|edition=8th}}</ref> This is achieved by the [[muscles of respiration]], through the contraction of the [[Thoracic diaphragm|diaphragm]], and the [[intercostal muscles]] which pull the [[rib cage]] upwards as shown in the diagram.<ref>{{cite book |last1= Tortora |first1= Gerard J. |last2=Anagnostakos|first2=Nicholas P.| title=Principles of anatomy and physiology |pages=567|edition= Fifth |location= New York |publisher= Harper & Row, Publishers|publication-date= 1987 |isbn= 978-0-06-350729-6 |year= 1987 }}</ref> During [[exhalation|breathing out]] the muscles relax, returning the lungs to their resting position.<ref name=tortora1 >{{cite book |last1= Tortora |first1= Gerard J. |last2=Anagnostakos|first2=Nicholas P.| title=Principles of anatomy and physiology |pages=556–582|edition= Fifth |location= New York |publisher= Harper & Row, Publishers|publication-date= 1987 |isbn= 978-0-06-350729-6 |year= 1987 }}</ref> At this point the lungs contain the [[functional residual capacity]] (FRC) of air, which, in the adult human, has a volume of about 2.5–3.0 litres.<ref name=tortora1 />

During [[Hyperpnoea|heavy breathing]] as in [[exertion]], a large number of [[Muscles of respiration#Accessory muscles|accessory muscles]] in the neck and abdomen are recruited, that during exhalation pull the ribcage down, decreasing the volume of the thoracic cavity.<ref name=tortora1 /> The FRC is now decreased, but since the lungs cannot be emptied completely there is still about a litre of residual air left.<ref name=tortora1 /> [[#Function testing|Lung function testing]] is carried out to evaluate [[lung volumes]] and capacities.

=== Protection ===
The lungs possess several characteristics which protect against infection. The respiratory tract is lined by [[respiratory epithelium]] or respiratory mucosa, with hair-like projections called [[cilia]] that beat rhythmically and carry [[mucus]]. This [[mucociliary clearance]] is an important defence system against air-borne infection.<ref name=pmid26475269/> The dust particles and bacteria in the inhaled air are caught in the mucosal surface of the airways, and are moved up towards the pharynx by the rhythmic upward beating action of the cilia.<ref name=PAWLINA /><ref name=DAVIDSONS2014 />{{rp|661–730}} The lining of the lung also secretes [[immunoglobulin A]] which protects against respiratory infections;<ref name=DAVIDSONS2014 /> [[goblet cells]] secrete mucus<ref name=PAWLINA /> which also contains several antimicrobial compounds such as [[defensin]]s, [[antiprotease]]s, and [[antioxidant]]s.<ref name=DAVIDSONS2014 /> A rare type of specialised cell called a '''pulmonary ionocyte''' that is suggested may regulate mucus viscosity has been described.<ref>{{Cite journal |display-authors=1 | doi = 10.1038/s41586-018-0393-7 | pmid = 30069044 | pmc = 6295155 | title = A revised airway epithelial hierarchy includes CFTR-expressing ionocytes | journal = Nature | volume = 560 | issue = 7718 | pages = 319–324 | year = 2018 | last1 = Montoro | first1 = Daniel T | last2 = Haber | first2 = Adam L | last3 = Biton | first3 = Moshe | last4 = Vinarsky | first4 = Vladimir | last5 = Lin | first5 = Brian | last6 = Birket | first6 = Susan E | last7 = Yuan | first7 = Feng | last8 = Chen | first8 = Sijia | last9 = Leung | first9 = Hui Min | last10 = Villoria | first10 = Jorge | last11 = Rogel | first11 = Noga | last12 = Burgin | first12 = Grace | last13 = Tsankov | first13 = Alexander M | last14 = Waghray | first14 = Avinash | last15 = Slyper | first15 = Michal | last16 = Waldman | first16 = Julia | last17 = Nguyen | first17 = Lan | last18 = Dionne | first18 = Danielle | last19 = Rozenblatt-Rosen | first19 = Orit | last20 = Tata | first20 = Purushothama Rao | last21 = Mou | first21 = Hongmei | last22 = Shivaraju | first22 = Manjunatha | last23 = Bihler | first23 = Hermann | last24 = Mense | first24 = Martin | last25 = Tearney | first25 = Guillermo J | last26 = Rowe | first26 = Steven M | last27 = Engelhardt | first27 = John F | last28 = Regev | first28 = Aviv | last29 = Rajagopal | first29 = Jayaraj| bibcode = 2018Natur.560..319M }}</ref><ref>{{Cite journal |display-authors=1 | doi = 10.1038/s41586-018-0394-6 | pmid = 30069046 | pmc = 6108322 | title = A single-cell atlas of the airway epithelium reveals the CFTR-rich pulmonary ionocyte | journal = Nature | volume = 560|issue = 7718 | pages = 377–381 | year = 2018 | last1 = Plasschaert | first1 = LW | last2 = Zillionis | first2 = R | last3 = Choo-Wing | first3 = R | last4 = Savova | first4 = V | last5 = Knehr | first5 = J | last6 = Roma | first6 = G | last7 = Klein | first7 = AM | last8 = Jaffe | first8 = AB| bibcode = 2018Natur.560..377P }}</ref><ref name="CFN">{{cite web |title=CF Study Finds New Cells Called Ionocytes Carrying High levels of CFTR Gene |url=https://cysticfibrosisnewstoday.com/2018/08/03/cf-study-finds-new-cells-ionocytes-carrying-high-cftr-levels/ |website=Cystic Fibrosis News Today |date=3 August 2018}}</ref> In addition, the lining of the lung also contains [[macrophage]]s, immune cells which engulf and destroy debris and microbes that enter the lung in a process known as [[phagocytosis]]; and [[dendritic cell]]s which present antigens to activate components of the [[adaptive immune system]] such as [[T cell]]s and [[B cell]]s.<ref name=DAVIDSONS2014 />

The size of the respiratory tract and the flow of air also protect the lungs from larger particles. Smaller particles deposit in the [[human mouth|mouth]] and behind the mouth in the [[Pharynx#Oropharynx|oropharynx]], and larger particles are trapped in [[nasal hair]] after inhalation.<ref name=DAVIDSONS2014 />

=== Other ===
In addition to their function in respiration, the lungs have a number of other functions. They are involved in maintaining [[homeostasis]], helping in the regulation of [[blood pressure]] as part of the [[renin–angiotensin system]]. The [[endothelium|inner lining]] of the blood vessels secretes [[angiotensin-converting enzyme]] (ACE) an [[enzyme]] that [[catalysis|catalyses]] the conversion of [[angiotensin I]] to [[angiotensin II]].<ref name=boron>{{cite book |author=Walter F. Boron |title=Medical Physiology: A Cellular And Molecular Approach |publisher=Elsevier/Saunders |year=2004 |isbn=978-1-4160-2328-9 |page=605}}</ref> The lungs are involved in the blood's [[acid–base homeostasis]] by expelling [[carbon dioxide]] when breathing.<ref name="Levitzky2013_1"/><ref name="PatientInfo">{{cite web|last1=Hoad-Robson|first1=Rachel|first2=Tim |last2=Kenny|title=The Lungs and Respiratory Tract|url=http://patient.info/health/the-lungs-and-respiratory-tract|website=Patient.info|publisher=[[Patient UK]]|access-date=11 February 2016|archive-url=https://web.archive.org/web/20150915174801/http://patient.info/health/the-lungs-and-respiratory-tract|archive-date=15 September 2015}}</ref>

The lungs also serve a protective role. Several blood-borne substances, such as a few types of [[prostaglandin]]s, [[leukotrienes]], [[serotonin]] and [[bradykinin]], are excreted through the lungs.<ref name=boron /> Drugs and other substances can be absorbed, modified or excreted in the lungs.<ref name=Levitzky2013_1 /><ref name="Smyth">{{cite book|last1=Smyth|first1=Hugh D.C.|title=Controlled pulmonary drug delivery|date=2011|publisher=Springer|location=New York|isbn=978-1-4419-9744-9|chapter=Chapter 2}}</ref> The lungs filter out small [[thrombus|blood clots]] from [[vein]]s and prevent them from entering [[artery|arteries]] and causing [[stroke]]s.<ref name=PatientInfo />

The lungs also play a pivotal role in [[speech]] by providing air and airflow for the creation of vocal sounds,<ref name=Levitzky2013_1 /><ref>{{cite web|last1=Mannell|first1=Robert|title=Introduction to Speech Production|url=http://clas.mq.edu.au/speech/phonetics/phonetics/introduction/|website=Macquarie University|access-date=8 February 2016}}</ref> and other [[paralanguage]] [[Paralanguage#Specific forms of paralinguistic respiration|communications]] such as [[sighing|sighs]] and [[gasp]]s.

Research suggests a role of the lungs in the production of blood platelets.<ref>{{Cite web | url=https://www.nih.gov/news-events/nih-research-matters/overlooked-role-lungs-blood-formation | title=An overlooked role for lungs in blood formation| date=2017-04-03}}</ref>