Editing Lung (section)

===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.