Editing Lung (section)

===Respiratory tract===
{{Main|Respiratory tract}}
[[File: Illu conducting passages.svg|thumb|upright|The lungs as main part of respiratory tract]]
The [[Respiratory tract#Lower respiratory tract|lower respiratory tract]] is part of the [[respiratory system]], and consists of the [[trachea]] and the structures below this including the lungs.<ref name="BERNELEVY">{{cite book|editor1=Stanton, Bruce M. |editor2=Koeppen, Bruce A.|title=Berne & Levy physiology|date=2008|publisher=Mosby/Elsevier|location=Philadelphia|isbn=978-0-323-04582-7|pages=418–422|edition=6th}}</ref> The trachea receives air from the [[pharynx]] and travels down to a place where it splits (the [[carina of trachea|carina]]) into a right and left primary [[bronchus]]. These supply air to the right and left lungs, splitting progressively into the secondary and tertiary bronchi for the lobes of the lungs, and into smaller and smaller bronchioles until they become the [[respiratory bronchiole]]s. These in turn supply air through [[alveolar duct]]s into the [[Pulmonary alveolus|alveoli]], where the [[gas exchange|exchange of gases]] take place.<ref name=BERNELEVY /> Oxygen [[inhalation|breathed in]], [[Molecular diffusion#Biology|diffuses]] through the walls of the alveoli into the enveloping [[capillaries]] and into the [[circulatory system|circulation]],<ref name="Pocock">{{cite book|last1=Pocock|first1=Gillian|last2=Richards|first2=Christopher D.|title=Human physiology : the basis of medicine|date=2006|publisher=Oxford University Press|location=Oxford|isbn=978-0-19-856878-0|pages=315–318|edition=3rd}}</ref> and carbon dioxide diffuses from the blood into the lungs to be [[exhalation|breathed out]].

Estimates of the total surface area of lungs vary from {{convert|50|to|75|sqm|sqft}};<ref name=BERNELEVY /><ref name="PAWLINA">{{cite book|last1=Pawlina|first1=W|title=Histology a Text & Atlas|date=2015|isbn=978-1-4511-8742-7|pages=670–678|publisher=Wolters Kluwer Health|edition=7th}}</ref> although this is often quoted in textbooks and the media being "the size of a tennis court",<ref name=PAWLINA /><ref>{{Cite web|title=Tennis Courts and Godzilla: A Conversation with Lung Biologist Thiennu Vu|url=https://www.ucsf.edu/news/2008/04/3797/tennis-courts-and-godzilla-conversation-lung-biologist-thiennu-vu|last=Miller|first=Jeff|date=11 April 2008|website=UCSF News & Media|language=en|access-date=2020-05-05}}</ref><ref>{{Cite web|title=8 Interesting Facts About Lungs|url=https://bronchiectasisnewstoday.com/social-clips/2016/10/14/8-curious-facts-about-lungs/4/|date=2016-10-17|website=Bronchiectasis News Today|language=en-US|access-date=2020-05-05}}</ref> it is actually less than half the size of a [[Tennis court|singles court]].<ref>{{cite book |author=Notter, Robert H. |title=Lung surfactants: basic science and clinical applications |publisher=Marcel Dekker |location=New York |year=2000 |pages=120 |isbn=978-0-8247-0401-8 |url=https://books.google.com/books?id=pAuiWvNHwZcC&q=area+tennis+court+alveoli&pg=PA120|access-date=2008-10-11}}</ref>

The bronchi in the [[conducting zone]] are reinforced with [[hyaline cartilage]] in order to hold open the airways. The bronchioles have no cartilage and are surrounded instead by [[Smooth muscle tissue|smooth muscle]].<ref name=PAWLINA />  Air is warmed to {{convert|37|°C|°F}}, [[humidity|humidified]] and cleansed by the conducting zone. [[Particulates|Particles]] from the air being removed by the [[cilia]] on the [[respiratory epithelium]] lining the passageways,<ref name="Ahmadi2013">{{cite book|author=Jiyuan Tu|author2=Kiao Inthavong|author3=Goodarz Ahmadi|title=Computational fluid and particle dynamics in the human respiratory system|url=https://archive.org/details/computationalflu00tuji_610|url-access=limited|date=2013|publisher=Springer|location=Dordrecht|isbn=9789400744875|pages=[https://archive.org/details/computationalflu00tuji_610/page/n37 23]–24|edition=1st}}</ref> in a process called [[mucociliary clearance]].

[[Pulmonary stretch receptors]] in the smooth muscle of the airways initiate a [[reflex]] known as the [[Hering–Breuer reflex]] that prevents the lungs from over-inflation, during forceful inspiration.