View source for Lung

{{short description|Primary organ of the respiratory system}}
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{{Anatomy terms}}
{{Use dmy dates|date=September 2025}}
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{{Use Australian English|date=September 2016}}
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{{Infobox anatomy
| Name        =Lung 
| Latin       = pulmo
| Greek       = πνεύμων (pneumon)
| Image       = Lungs diagram detailed.svg
| Caption     = Diagram of the human lungs with the respiratory tract visible, and different colours for each lobe
| Width       =
| Image2      = heart-and-lungs.jpg
| Caption2    = The human lungs flank the heart and great vessels in the chest cavity.
| Precursor   =
| System      = [[Respiratory system]]
| Artery      = [[Pulmonary artery]]
| Vein        = [[Pulmonary vein]]
| Nerve       =
| Lymph       =
}}

The '''lungs''' are the primary [[Organ (biology)|organs]] of the [[respiratory system]] in many animals, including humans. In [[mammal]]s and most other [[tetrapod]]s, two lungs are located near the [[Vertebral column|backbone]] on either side of the [[heart]]. Their function in the respiratory system is to extract oxygen from the atmosphere and transfer it into the bloodstream, and to release [[carbon dioxide]] from the bloodstream into the atmosphere, in a process of [[gas exchange]]. Respiration is driven by different muscular systems in different species. Mammals, reptiles and birds use their musculoskeletal systems to support and foster breathing. In early tetrapods, air was driven into the lungs by the pharyngeal muscles via [[buccal pumping]], a mechanism still seen in amphibians. In humans, the primary muscle that drives breathing is the [[Thoracic diaphragm|diaphragm]]. The lungs also provide airflow that makes [[Animal communication#Auditory|vocalisation]] including [[speech]] possible.

Humans have two lungs, a right lung and a left lung. They are situated within the thoracic cavity of the [[chest]]. The right lung is bigger than the left, and the left lung shares space in the chest with the heart. The lungs together weigh approximately 1.3 kilograms (2.9 lb), and the right is heavier. The lungs are part of the [[lower respiratory tract]] that begins at the [[trachea]] and branches into the [[bronchi]] and [[bronchiole]]s, which receive air [[Inhalation|breathed in]] via the [[conducting zone]]. These divide until air reaches microscopic [[Pulmonary alveolus|alveoli]], where gas exchange takes place. Together, the lungs contain approximately 2,400 kilometers (1,500 mi) of airways and 300 to 500 million alveoli. Each lung is enclosed within a [[pleural sac]] of two [[pleurae]] which allows the inner and outer walls to slide over each other whilst breathing takes place, without much friction. The inner [[visceral pleura]] divides each lung as [[Lung#Fisssures|fissure]]s into sections called lobes. The right lung has three lobes and the left has two. The lobes are further divided into [[bronchopulmonary segment]]s and lobules. The lungs have a unique blood supply, receiving deoxygenated blood sent from the heart to receive oxygen (the [[pulmonary circulation]]) and a separate supply of oxygenated blood (the [[bronchial circulation]]).

The tissue of the lungs can be affected by several [[respiratory disease]]s including [[pneumonia]] and [[lung cancer]]. Chronic diseases such as [[chronic obstructive pulmonary disease]] and [[emphysema]] can be related to smoking or exposure to harmful substances. Diseases such as bronchitis can also affect the respiratory tract. Medical terms related to the lung often begin with pulmo-, from the Latin pulmonarius (of the lungs) as in pulmonology, or with pneumo- (from Greek πνεύμων "lung") as in pneumonia.

In [[embryonic development]], the lungs begin to develop as an outpouching of the [[foregut]], a tube which goes on to form the upper part of the [[digestive system]]. When the lungs are formed the [[fetus]] is held in the fluid-filled [[amniotic sac]] and so they do not function to breathe. Blood is also diverted from the lungs through the [[ductus arteriosus]]. [[Adaptation to extrauterine life#Breathing and circulation|At birth]] however, air begins to pass through the lungs, and the diversionary duct closes so that the lungs can begin to respire. The lungs only fully develop in early childhood.

== Structure ==
=== Anatomy ===
[[File:Illu bronchi lungs.jpg|upright=1.5|thumb|right]]
In humans, the lungs are located in the [[thoracic cavity|chest]] on either side of the [[heart]] in the [[rib cage]]. They are conical in shape with a narrow rounded '''apex''' at the top, and a broad concave '''base''' that rests on the convex surface of the [[thoracic diaphragm|diaphragm]].<ref name=GRAYS /> The apex of the lung extends into the root of the neck, reaching shortly above the level of the [[sternum|sternal]] end of the [[Rib cage#Bones|first rib]]. The lungs stretch from close to the [[vertebral column|backbone]] in the rib cage to the front of the [[thorax|chest]] and downwards from the lower part of the trachea to the diaphragm.<ref name=GRAYS>{{cite book|last1=Drake |first1=Richard L.|last2=Vogl|first2=Wayne|last3=Mitchell|first3=Adam W.M.|title=Gray's anatomy for students|date=2014|publisher=[[Churchill Livingstone]]/[[Elsevier]]|location=Edinburgh|isbn=978-0-7020-5131-9|pages=167–174|edition=3rd}}</ref> 

The left lung shares space with the heart, and has an indentation in its border called the '''cardiac notch of the left lung''' to accommodate this.<ref name="Betts">{{cite book|last1=Betts|first1=J. Gordon|title=Anatomy & physiology|date=2013|isbn=978-1-938168-13-0|url=http://cnx.org/content/m46676/latest/?collection=col11496/latest|access-date=11 August 2014|pages=787–846|publisher=OpenStax College, Rice University }}</ref><ref name="GRAYS40TH" /> The front and outer sides of the lungs face the ribs, which make light indentations on their surfaces. The medial surfaces of the lungs face towards the centre of the chest, and lie against the heart, [[great vessels]], and the [[carina of trachea|carina]] where the trachea divides into the two main bronchi.<ref name="GRAYS40TH" />  The '''cardiac impression''' is an indentation formed on the surfaces of the lungs where they rest against the heart.

Both lungs have a central recession called the [[root of the lung|hilum]], where the [[blood vessel]]s and [[Bronchus|airways]] pass into the lungs making up the [[root of the lung]].<ref name="Moore K"/> There are also [[tracheobronchial lymph node|bronchopulmonary lymph nodes]] on the hilum.<ref name="GRAYS40TH" />

The lungs are surrounded by the [[pulmonary pleurae]]. The pleurae are two [[serous membrane]]s; the outer [[parietal pleura]] lines the inner wall of the [[rib cage]] and the inner [[visceral pleura]] directly lines the surface of the lungs. Between the pleurae is a [[potential space]] called the [[pleural cavity]] containing a thin layer of lubricating [[pleural fluid]].

====Lobes====
{| class="wikitable" style="float:right;margin-left:10px"
|+ Lobes and bronchopulmonary segments<ref name="pmid1" />
|-
! Right lung !! Left lung
|-
| '''Upper'''
* Apical
* Posterior
* Anterior
'''Middle'''
* Lateral
* Medial
'''Lower'''
* Superior
* Medial
* Anterior
* Lateral
* Posterior 
|| '''Upper'''
* Apicoposterior
* Anterior
'''Lingula'''
* Superior
* Inferior
'''Lower'''
* Superior
* Anteriomedial
* Lateral
* Posterior
|}

Each lung is divided into sections called lobes by the reflections (infoldings) of the visceral pleura as fissures. Lobes are divided into segments, and segments have further divisions as lobules. There are three lobes in the right lung and two lobes in the left lung.

====Fissures====
The fissures are formed in early [[prenatal development]] by invaginations of the visceral pleura that divide the lobar bronchi, and section the lungs into lobes that helps in their expansion.<ref name="Koster">{{cite journal |last1=Koster |first1=TD |last2=Slebos |first2=DJ |title=The fissure: interlobar collateral ventilation and implications for endoscopic therapy in emphysema. |journal=International Journal of Chronic Obstructive Pulmonary Disease |date=2016 |volume=11 |pages=765–73 |doi=10.2147/COPD.S103807 |pmid=27110109|pmc=4835138 |doi-access=free }}</ref><ref name="Lung fissures">{{cite web|last1=Hacking|first1=Craig|last2=Knipe|first2=Henry|title=Lung fissures|url=http://radiopaedia.org/articles/lung-fissures|website=Radiopaedia|date=22 March 2013 |access-date=8 February 2016}}</ref> The right lung is divided into three lobes by a '''horizontal fissure''', and an '''oblique fissure'''. The left lung is divided into two lobes by an oblique fissure which is closely aligned with the oblique fissure in the right lung. In the right lung the upper horizontal fissure, separates the upper (superior) lobe from the middle lobe. The lower, oblique fissure separates the lower lobe from the middle and upper lobes.<ref name=GRAYS /><ref name="Lung fissures" />

[[Lung#Variation|Variations]] in the fissures are fairly common being either incompletely formed or present as an extra fissure as in the [[azygos lobe|azygos fissure]], or absent. Incomplete fissures are responsible for [[collateral ventilation|interlobar collateral ventilation]], airflow between lobes which is unwanted in some [[lung volume reduction]] procedures.<ref name="Koster" />

====Segments====
The primary bronchi enter the lungs at the hilum and initially branch into [[Bronchus|secondary bronchi]] also known as lobar bronchi that supply air to each lobe of the lung. The lobar bronchi branch into [[Bronchus|tertiary bronchi]] also known as segmental bronchi and these supply air to the further divisions of the lobes known as [[bronchopulmonary segment]]s. Each bronchopulmonary segment has its own (segmental) bronchus and [[artery|arterial supply]].<ref>{{cite web|last1=Jones|first1=Jeremy|title=Bronchopulmonary segmental anatomy {{!}} Radiology Reference Article {{!}} Radiopaedia.org|url=https://radiopaedia.org/articles/bronchopulmonary-segmental-anatomy-1|website=radiopaedia.org|date=May 2011 |language=en}}</ref> Segments for the left and right lung are shown in the table.<ref name="pmid1">{{cite journal|last1=Arakawa|first1=H|last2=Niimi|first2=H|last3=Kurihara|first3=Y|last4=Nakajima|first4=Y|last5=Webb|first5=WR|title=Expiratory high-resolution CT: diagnostic value in diffuse lung diseases|journal=American Journal of Roentgenology|date=December 2000|volume=175|issue=6|pages=1537–1543|pmid=11090370|doi=10.2214/ajr.175.6.1751537}}</ref> The segmental anatomy is useful clinically for localising disease processes in the lungs.<ref name="pmid1" /> A segment is a discrete unit that can be surgically removed without seriously affecting surrounding tissue.<ref name="Harper">{{cite book|last1=Tortora |first1=Gerard |title=Principles of anatomy and physiology |date=1987 |publisher=Harper and Row| location=New York |isbn=978-0-06-350729-6 |page=564 |edition=5th}}</ref>
{{multiple image|caption_align=center|header_align=center|align=center
 | total_width = 480
 | image1 = Gray973.png
 | alt1 = The left lung
 | image2 = Gray972.png
 | alt2 = The right lung
 | footer = The left lung (left) and right lung (right). The lobes of the lungs can be seen, and the central [[root of the lung]] is also present.
}}
{{Clear}}

=== Right lung ===
The right lung has both more lobes and segments than the left. It is divided into three lobes, an upper, middle, and a lower lobe by two fissures, one oblique and one horizontal.<ref name="Chaudhry">{{cite book|vauthors=Chaudhry R, Bordoni B|title=StatPearls [Internet]|chapter=Anatomy, Thorax, Lungs|date=Jan 2019 |pmid=29262068}}</ref> The upper, horizontal fissure, separates the upper from the middle lobe. It begins in the lower oblique fissure near the posterior border of the lung, and, running horizontally forward, cuts the anterior border on a level with the [[sternum|sternal]] end of the fourth [[costal cartilage]]; on the [[mediastinal]] surface it may be traced back to the [[root of the lung|hilum]].<ref name=GRAYS /> The lower, oblique fissure, separates the lower from the middle and upper lobes and is closely aligned with the oblique fissure in the left lung.<ref name=GRAYS /><ref name="Lung fissures" />

The mediastinal surface of the right lung is indented by a number of nearby structures. The heart sits in an impression called the cardiac impression. Above the hilum of the lung is an arched groove for the [[azygos vein]], and above this is a wide groove for the [[superior vena cava]] and right [[brachiocephalic vein]]; behind this, and close to the top of the lung is a groove for the [[brachiocephalic artery]]. There is a groove for the [[esophagus|oesophagus]] behind the hilum and the [[pulmonary ligament]], and near the lower part of the oesophageal groove is a deeper groove for the [[inferior vena cava]] before it enters the heart.<ref name="GRAYS40TH">{{cite book|last1=Standring|first1=Susan|editor1-last=Borley|editor1-first=Neil R.|title=Gray's Anatomy: The Anatomical Basis of Clinical Practice|date=2008|publisher=[[Churchill Livingstone]]/[[Elsevier]]|location=Edinburgh|isbn=978-0-443-06684-9|edition=40th|pages=992–1000|url=https://books.google.com/books?id=kvhkPQAACAAJ}} [https://archive.org/stream/GraysAnatomy40thEd_201403/Gray's%20Anatomy%20-%2040th%20Ed_djvu.txt Alt URL]</ref>

The weight of the right lung varies between individuals, with a standard [[reference range]] in men of {{convert|155-720|g|lb|abbr=on}}<ref name="MolinaDiMaio2012">{{cite journal |last1=Molina |first1=D. Kimberley |last2=DiMaio |first2=Vincent J.M. |title=Normal Organ Weights in Men |journal=The American Journal of Forensic Medicine and Pathology |date=December 2012 |volume=33 |issue=4 |pages=368–372 |doi=10.1097/PAF.0b013e31823d29ad |pmid=22182984 |s2cid=32174574 }}</ref> and in women of {{convert|100-590|g|lb|abbr=on}}.<ref name="MolinaDiMaio2015">{{cite journal |last1=Molina |first1=D. Kimberley |last2=DiMaio |first2=Vincent J. M. |title=Normal Organ Weights in Women |journal=The American Journal of Forensic Medicine and Pathology |date=September 2015 |volume=36 |issue=3 |pages=182–187 |doi=10.1097/PAF.0000000000000175 |pmid=26108038 |s2cid=25319215 }}</ref>

=== Left lung ===
The left lung is divided into two lobes, an upper and a lower lobe, by the oblique fissure, which extends from the [[rib|costal]] to the mediastinal surface of the lung both above and below the [[Hilum of lung|hilum]].<ref name=GRAYS /> The left lung, unlike the right, does not have a middle lobe, though it does have a [[Homology (biology)|homologous]] feature, a projection of the upper lobe termed the '''lingula'''.  Its name means "little tongue". The lingula on the left lung serves as an anatomic parallel to the middle lobe on the right lung, with both areas being predisposed to similar infections and anatomic complications.<ref>{{cite journal|pmid=21324708|year=2011|last1=Yu|first1=J.A.|title=Lady Windermere revisited: Treatment with thoracoscopic lobectomy/segmentectomy for right middle lobe and lingular bronchiectasis associated with non-tuberculous mycobacterial disease|journal=European Journal of Cardio-Thoracic Surgery|volume=40|issue=3|pages=671–675|last2=Pomerantz|first2=M|last3=Bishop|first3=A|last4=Weyant|first4=M.J.|last5=Mitchell|first5=J.D.|doi=10.1016/j.ejcts.2010.12.028|doi-access=free}}</ref><ref>{{cite journal|pmid=14718418|year=2004|last1=Ayed|first1=A.K.|title=Resection of the right middle lobe and lingula in children for middle lobe/lingula syndrome|journal=Chest|volume=125|issue=1|pages=38–42|doi=10.1378/chest.125.1.38|s2cid=45666843}}</ref> There are two [[bronchopulmonary segment]]s of the lingula: superior and inferior.<ref name=GRAYS />

The mediastinal surface of the left lung has a large ''cardiac impression'' where the heart sits. This is deeper and larger than that on the right lung, at which level the heart projects to the left.<ref name=GRAYS40TH />

On the same surface, immediately above the hilum, is a well-marked curved groove for  the [[aortic arch]], and a groove below it for the [[descending aorta]]. The [[subclavian artery|left subclavian artery]], a branch off the aortic arch, sits in a groove from the arch to near the apex of the lung. A shallower groove in front of the artery and near the edge of the lung, lodges the left [[brachiocephalic vein]]. The oesophagus may sit in a wider shallow impression at the base of the lung.<ref name=GRAYS40TH />

By standard [[reference range]], the weight of the left lung is {{convert|110-675|g|lb|abbr=on}}<ref name="MolinaDiMaio2012"/> in men and {{convert|105-515|g|lb|abbr=on}} in women.<ref name="MolinaDiMaio2015"/>

===Illustrations===
<gallery>
File:CT-Thorax-5.0-B70f-Lungs.jpg|Chest CT (axial lung window)
File:COR-2-STND-CHEST-LUNGS.jpg|Chest CT (coronal lung window)
File:CT-Thorax-5.0-B70f.ogg|Chest CT (axial lung window)
File:COR-2-STND-CHEST.ogg|Chest CT (coronal lung window)
File:Meet the lungs.webm|"Meet the lungs" from [[Khan Academy]]
File:MP1 Pulmonology.webm|Pulmonology video
File:Lobes of the Lung ogg.mov (1).ogg|3D anatomy of the lung lobes and fissures.
</gallery>

=== Microanatomy ===
[[File:Lung structure normal.jpg|thumb|upright=1.25|Cross-sectional detail of the lung]]
The lungs are part of the [[lower respiratory tract]], and accommodate the bronchial airways when they branch from the trachea. The bronchial airways terminate in [[pulmonary alveolus|alveoli]] which make up the functional tissue ([[lung parenchyma|parenchyma]]) of the lung, and veins, arteries, nerves, and [[lymphatic vessel]]s.<ref name=GRAYS40TH /><ref>{{cite book |vauthors=Young B, Lowe JS, Stevens A, Heath JW |others=Deakin PJ (illust) |title=Wheater's functional histology: a text and colour atlas|url=https://archive.org/details/wheatersfunction00youn |url-access=limited |date=2006|publisher=Churchill Livingstone/Elsevier|location=[Edinburgh?]|isbn=978-0-443-06850-8|pages=[https://archive.org/details/wheatersfunction00youn/page/n531 234]–250|edition=5th}}</ref> The trachea and bronchi have plexuses of [[lymph capillary|lymph capillaries]] in their mucosa and submucosa. The smaller bronchi have a single layer of lymph capillaries, and they are absent in the alveoli.<ref name="Theodora">{{cite web|url=https://theodora.com/anatomy/the_lymphatic_system.html|title=The Lymphatic System – Human Anatomy|access-date=8 September 2017}}</ref> The lungs are supplied with the largest lymphatic drainage system of any other organ in the body.<ref name="Saladin">{{cite book |last1=Saladin |first1=Kenneth S. |title=Human anatomy |date=2011 |publisher=McGraw-Hill |location=New York |isbn=978-0-07-122207-5 |page=634 |edition=3rd}}</ref> Each lung is surrounded by a [[serous membrane]] of [[pulmonary pleurae|visceral pleura]], which has an underlying layer of [[loose connective tissue]] attached to the substance of the lung.<ref name="Dorlands1077">{{cite book|author1=Dorland|author-link1=Dorland's medical reference works|title=Dorland's Illustrated Medical Dictionary|publisher=Elsevier|isbn=978-1-4160-6257-8|edition=32nd|page=1077|url=https://books.google.com/books?id=mNACisYwbZoC&q=1077&pg=PA1077|access-date=11 February 2016|date=2011-06-09}}</ref>

====Connective tissue====
[[File:STD 190219 SWITCH Tissue 2 93x CMLE 20SNR 50IT Elastin.png|thumb|Thick [[elastic fibres]] from the [[visceral pleura]] (outer lining) of lung]]
[[File:Fibers of Collagen Type I - TEM.jpg|thumb|[[Transmission electron microscopy|TEM]] image of [[collagen fibres]] in a cross sectional slice of mammalian lung tissue]]
The connective tissue of the lungs is made up of [[Elastic fiber|elastic]] and [[collagen fibres]] that are interspersed between the capillaries and the alveolar walls. [[Elastin]] is the key [[protein]] of the [[extracellular matrix]] and is the main component of the [[elastic fibres]].<ref name="Mithieux">{{cite book |doi=10.1016/S0065-3233(05)70013-9 |chapter=Elastin |title=Fibrous Proteins: Coiled-Coils, Collagen and Elastomers |series=Advances in Protein Chemistry |year=2005 |last1=Mithieux |first1=Suzanne M. |last2=Weiss |first2=Anthony S. |volume=70 |pages=437–461 |pmid=15837523 |isbn=978-0-12-034270-9 }}</ref> Elastin gives the necessary elasticity and resilience required for the persistent stretching involved in breathing, known as [[lung compliance]]. It is also responsible for the [[elastic recoil]] needed. Elastin is more concentrated in areas of high stress such as the openings of the alveoli, and alveolar junctions.<ref name="Mithieux"/> The connective tissue links all the alveoli to form the lung parenchyma which has a sponge-like appearance. The alveoli have interconnecting air passages in their walls known as the [[pores of Kohn]].<ref name=Pocock/>

====Respiratory epithelium====
{{Main|Respiratory epithelium}}
All of the lower respiratory tract including the trachea, bronchi, and bronchioles is lined with [[respiratory epithelium]]. This is a [[cilium|ciliated]] epithelium interspersed with [[goblet cell]]s which produce [[mucin]] the main component of [[mucus]], ciliated cells, [[airway basal cell|basal cells]], and in the [[terminal bronchiole]]s–[[club cells]] with actions similar to basal cells, and [[macrophage]]s. The epithelial cells, and the [[submucosal gland]]s throughout the respiratory tract secrete [[Mucus#Respiratory system|airway surface liquid]] (ASL), the composition of which is tightly regulated and determines how well [[mucociliary clearance]] works.<ref>{{cite journal | last1 = Stanke | first1 = F | year = 2015 | title = The Contribution of the Airway Epithelial Cell to Host Defense | journal = Mediators Inflamm | volume = 2015 | article-number = 463016 | doi = 10.1155/2015/463016 | pmid=26185361 | pmc=4491388| doi-access = free }}</ref>

[[Neuroendocrine cell#Pulmonary neuroendocrine cells|Pulmonary neuroendocrine cells]] are found throughout the respiratory epithelium including the alveolar epithelium,<ref name="Van Lommel">{{cite journal |last1=Van Lommel |first1=A |title=Pulmonary neuroendocrine cells (PNEC) and neuroepithelial bodies (NEB): chemoreceptors and regulators of lung development. |journal=Paediatric Respiratory Reviews |date=June 2001 |volume=2 |issue=2 |pages=171–6 |pmid=12531066|doi=10.1053/prrv.2000.0126 }}</ref> though they only account for around 0.5 percent of the total epithelial population.<ref name="Garg">{{cite book |doi=10.1016/bs.ctdb.2018.12.002 |chapter=Consider the lung as a sensory organ: A tip from pulmonary neuroendocrine cells |title=Organ Development |series=Current Topics in Developmental Biology |year=2019 |last1=Garg |first1=Ankur |last2=Sui |first2=Pengfei |last3=Verheyden |first3=Jamie M. |last4=Young |first4=Lisa R. |last5=Sun |first5=Xin |volume=132 |pages=67–89 |pmid=30797518 |isbn=978-0-12-810489-7 |s2cid=73489416 }}</ref> PNECs are innervated airway epithelial cells that are particularly focused at airway junction points.<ref name="Garg"/> These cells can produce serotonin, dopamine, and norepinephrine, as well as polypeptide products. Cytoplasmic processes from the pulmonary neuroendocrine cells extend into the airway lumen where they may sense the composition of inspired gas.<ref name="Weinberger">{{cite book |last1=Weinberger |first1=S |last2=Cockrill |first2=B |last3=Mandel |first3=J |title=Principles of pulmonary medicine |date=2019 |isbn=978-0-323-52371-4 |page=67 |publisher=Elsevier |edition=Seventh}}</ref>

====Bronchial airways====
In the bronchi there are incomplete [[Trachea#Structure|tracheal rings]] of [[cartilage]] and smaller plates of cartilage that keep them open.<ref name="GuytonHall">{{cite book|last1=Hall|first1=John|title=Guyton and Hall textbook of medical physiology|date=2011|publisher=Saunders/Elsevier|location=Philadelphia|isbn=978-1-4160-4574-8|edition=12th}}</ref>{{rp|472}} Bronchioles are too narrow to support cartilage and their walls are of [[smooth muscle]], and this is largely absent in the narrower [[respiratory bronchiole]]s which are mainly just of epithelium.<ref name="GuytonHall"/>{{rp|472}} The absence of cartilage in the terminal bronchioles gives them an alternative name of ''membranous bronchioles''.<ref name="Abbott">{{cite journal |last1=Abbott |first1=Gerald F. |last2=Rosado-de-Christenson |first2=Melissa L. |last3=Rossi |first3=Santiago E. |last4=Suster |first4=Saul |title=Imaging of Small Airways Disease |journal=Journal of Thoracic Imaging |volume=24 |issue=4 |pages=285–298 |doi=10.1097/RTI.0b013e3181c1ab83 |pmid=19935225 |date=November 2009|s2cid=10249069 |doi-access=free }}</ref>
[[File:Secondary-pulmonary-lobule-illustration.jpg|thumb|A lobule of the lung enclosed in septa and supplied by a terminal bronchiole that branches into the respiratory bronchioles. Each respiratory bronchiole supplies the alveoli held in each acinus accompanied by a pulmonary artery branch.]]

====Respiratory zone====
The conducting zone of the respiratory tract ends at the terminal bronchioles when they branch into the respiratory bronchioles. This marks the beginning of the terminal respiratory unit called the '''acinus''' which includes the respiratory bronchioles, the alveolar ducts, [[alveolar sac]]s, and alveoli.<ref name ="Weinberger2">{{Cite book|title=Principles of Pulmonary Medicine|last=Weinberger|first=Steven|publisher=Elsevier|year=2019|isbn=978-0-323-52371-4|page=2}}</ref> An acinus measures up to 10&nbsp;mm in diameter.<ref name=Hochhegger/> A '''primary pulmonary lobule''' is the part of the lung distal to the respiratory bronchiole.<ref name="Elsevier-2021">{{Cite book |title=Gray's Anatomy: the anatomical basis of clinical practice |date=2021 |publisher=Elsevier |isbn=978-0-7020-7705-0 |editor-last=Gray |editor-first=Henry |edition=42nd |location=Amsterdam |page=1028 |editor-last2=Standring |editor-first2=Susan |editor-last3=Anhand |editor-first3=Neel}}</ref> Thus, it includes the alveolar ducts, sacs, and alveoli but not the respiratory bronchioles.<ref name=Goel>{{Cite web|url=https://radiopaedia.org/articles/primary-pulmonary-lobule?lang=gb|title=Primary pulmonary lobule|last=Goel|first=A|date=28 February 2010 |access-date=12 July 2019}}</ref> 

The unit described as the '''secondary pulmonary lobule''' is the lobule most referred to as the '''pulmonary lobule''' or '''respiratory lobule'''.<ref name="GuytonHall"/>{{rp|489}}<ref name=Gaillard>{{Cite web|url=https://radiopaedia.org/articles/secondary-pulmonary-lobule?lang=gb|title=Secondary pulmonary lobule|last1=Gilcrease-Garcia|first1=B|last2=Gaillard|first2=Frank|website=radiopaedia.org|date=27 February 2010 |access-date=10 August 2019}}</ref> This lobule is a discrete unit that is the smallest component of the lung that can be seen without aid.<ref name="Elsevier-2021" /> The secondary pulmonary lobule is likely to be made up of between 30 and 50 primary lobules.<ref name=Goel/> The lobule is supplied by a terminal bronchiole that branches into respiratory bronchioles. The respiratory bronchioles supply the alveoli in each acinus and is accompanied by a [[pulmonary artery]] branch. Each lobule is enclosed by an interlobular septum. Each acinus is incompletely separated by an intralobular septum.<ref name=Hochhegger>{{Cite journal|last=Hochhegger|first=B|date=June 2019|title=Pulmonary Acinus: Understanding the Computed Tomography Findings from an Acinar Perspective.|journal=Lung|volume=197|issue=3|pages=259–265|pmid=30900014|doi=10.1007/s00408-019-00214-7|s2cid=84846517|hdl=10923/17852|hdl-access=free}}</ref>

The respiratory bronchiole gives rise to the alveolar ducts that lead to the alveolar sacs, which contain two or more alveoli.<ref name=Pocock/> The walls of the alveoli are extremely thin allowing a fast rate of [[Diffusion#Diffusion vs. bulk flow|diffusion]].  The alveoli have interconnecting small air passages in their walls known as the [[pores of Kohn]].<ref name=Pocock/>

====Alveoli====
{{Main|Pulmonary alveolus}}
[[File:Alveolus diagram.svg|thumb|right|Alveoli and their capillary networks]]
[[File:3D Medical Animation Bronchial Airways terminating ends.jpg|alt=A 3D Medical illustration showing different terminating ends of Bronchial airways connected to alveoili, lung parenchyma & lymphatic vessels.|thumb|3D medical illustration showing different terminating ends of bronchioles]]
Alveoli consist of two types of [[alveolar cell]] and an [[alveolar macrophage]]. The two types of cell are known as [[Pulmonary alveolus#Type I cells|type I]] and [[Pulmonary alveolus#Type II cells|type II cells]]<ref name=BERNELEVY /> (also known as pneumocytes).<ref name=GRAYS40TH /> Types I and II make up the walls and [[alveolar septum|alveolar septa]]. Type I cells provide 95% of the surface area of each alveoli and are flat ("[[squamous epithelial cell|squamous]]"), and Type II cells generally cluster in the corners of the alveoli and have a cuboidal shape.<ref name=PAWLINA /> Despite this, cells occur in a roughly equal ratio of 1:1 or 6:4.<ref name=BERNELEVY /><ref name=PAWLINA />

Type I are [[epithelium|squamous epithelial cells]] that make up the alveolar wall structure. They have extremely thin walls that enable an easy gas exchange.<ref name=BERNELEVY /> These type I cells also make up the alveolar septa which separate each alveolus. The septa consist of an epithelial lining and associated [[basement membrane]]s.<ref name=PAWLINA /> Type I cells are not able to divide, and consequently rely on [[cellular differentiation|differentiation]] from Type II cells.<ref name=PAWLINA />

Type II are larger and they line the alveoli and produce and secrete epithelial lining fluid, and [[pulmonary surfactant|lung surfactant]].<ref name=pmid26475269>{{cite journal |last1=Srikanth |first1=Lokanathan |last2=Venkatesh |first2=Katari |last3=Sunitha |first3=Manne Mudhu |last4=Kumar |first4=Pasupuleti Santhosh |last5=Chandrasekhar |first5=Chodimella |last6=Vengamma |first6=Bhuma |last7=Sarma |first7=Potukuchi Venkata Gurunadha Krishna |title=In vitro generation of type-II pneumocytes can be initiated in human CD34+ stem cells |journal=Biotechnology Letters |date=16 October 2015 |volume=38 |issue=2 |pages=237–242 |doi=10.1007/s10529-015-1974-2 |pmid=26475269 |s2cid=17083137 }}</ref><ref name=BERNELEVY /> Type II cells are able to divide and differentiate to Type I cells.<ref name=PAWLINA />

The [[alveolar macrophage]]s have an important role in the [[immune system]]. They remove substances which deposit in the alveoli including loose red blood cells that have been forced out from blood vessels.<ref name=PAWLINA />

====Microbiota====
{{Main|Lung microbiota}}
There is a large presence of [[microorganism]]s in the lungs known as the [[lung microbiota]] that interacts with the airway epithelial cells; an interaction of probable importance in maintaining homeostasis. The [[Microbiota#Humans|microbiota]] is complex and dynamic in healthy people, and altered in diseases such as [[asthma]] and [[COPD]]. For example significant changes can take place in COPD following infection with [[rhinovirus]].<ref name="Hiemstra">{{cite journal |last1=Hiemstra |first1=PS |last2=McCray PB |first2=Jr |last3=Bals |first3=R |title=The innate immune function of airway epithelial cells in inflammatory lung disease. |journal=The European Respiratory Journal |date=April 2015 |volume=45 |issue=4 |pages=1150–62 |doi=10.1183/09031936.00141514 |pmid=25700381|pmc=4719567 }}</ref> [[Fungus|Fungal]] [[genera]] that are commonly found as [[mycobiota]] in the microbiota include ''[[Candida (fungus)|Candida]]'', ''[[Malassezia]]'', ''[[Saccharomyces]]'', and ''[[Aspergillus]]''.<ref name="Cui">{{cite journal |vauthors=Cui L, Morris A, Ghedin E |title=The human mycobiome in health and disease |journal=Genome Med |volume=5 |issue=7 |page=63 |date=2013 |pmid=23899327 |pmc=3978422 |doi=10.1186/gm467 |url= |doi-access=free }}</ref><ref name="Richardson">{{cite journal |last1=Richardson |first1=M |last2=Bowyer |first2=P |last3=Sabino |first3=R |title=The human lung and Aspergillus: You are what you breathe in? |journal=Medical Mycology |date=1 April 2019 |volume=57 |issue=Supplement_2 |pages=S145–S154 |doi=10.1093/mmy/myy149 |pmid=30816978|pmc=6394755 }}</ref>

===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 |page=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=978-94-007-4487-5|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.

===Blood supply===
{{main |Pulmonary circulation}}
[[File:3D CT of thorax, annotated.jpg|thumb|upright=1.5|[[3D rendering]] of a [[high-resolution computed tomography|high-resolution CT scan]] of the [[thorax]]. The anterior thoracic wall, the airways and the pulmonary vessels anterior to the [[root of the lung]] have been digitally removed in order to visualise the different levels of the [[pulmonary circulation]].]]
The lungs have a dual blood supply provided by a [[bronchial circulation|bronchial]] and a [[pulmonary circulation]].<ref name="Moore K">{{cite book |last1=Moore |first1=K |title=Clinically oriented anatomy |date=2018 |isbn=978-1-4963-4721-3 |pages=333–339 |publisher=Wolters Kluwer |edition=8th}}</ref> The [[bronchial circulation]] supplies oxygenated blood to the airways of the lungs, through the [[bronchial artery|bronchial arteries]] that leave the [[aorta]].  There are usually three arteries, two to the left lung and one to the right, and they branch alongside the bronchi and bronchioles.<ref name=BERNELEVY /> The [[pulmonary circulation]] carries deoxygenated blood from the heart to the lungs and returns the oxygenated blood to the heart to supply the rest of the body.<ref name=BERNELEVY />

The blood volume of the lungs is about 450 millilitres on average, about 9% of the total blood volume of the entire circulatory system. This quantity can easily fluctuate from between one-half and twice the normal volume.  Also, in the event of blood loss through hemorrhage, blood from the lungs can partially compensate by automatically transferring to the systemic circulation.<ref>{{Cite book|title=Medical Physiology|last1=Guyton|first1=A|last2=Hall|first2=J|year=2011|isbn=978-1-4160-4574-8|pages=478|publisher=Saunders/Elsevier }}</ref>

=== Nerve supply ===
The lungs are supplied by nerves of the [[autonomic nervous system]]. Input from the [[parasympathetic nervous system]] occurs via the [[vagus nerve]].<ref name="Moore K"/> When stimulated by [[acetylcholine]], this causes constriction of the smooth muscle lining the bronchus and bronchioles, and increases the secretions from glands.<ref name="Levitzky2013_2">{{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 2. Mechanics of Breathing |edition=8th}}</ref>{{page needed |date=January 2018}} The lungs also have a sympathetic tone from [[norepinephrine]] acting on the [[Beta-2 adrenergic receptor|beta 2 adrenoceptors]] in the respiratory tract, which causes [[bronchodilation]].<ref name="Johnson">{{cite journal | vauthors = Johnson M | title = Molecular mechanisms of beta(2)-adrenergic receptor function, response, and regulation | journal = The Journal of Allergy and Clinical Immunology | volume = 117 | issue = 1 | pages = 18–24; quiz 25 | date = January 2006 | pmid = 16387578 | doi = 10.1016/j.jaci.2005.11.012 | doi-access = free }}</ref>

The action of breathing takes place because of nerve signals sent by the [[respiratory center]] in the [[brainstem]], along the [[phrenic nerve]] from the [[cervical plexus]] to the diaphragm.<ref>{{Cite book|title=Principles of Anatomy & Physiology|last1=Tortora|first1=G|last2=Derrickson|first2=B|publisher=Wiley|year=2011|isbn=978-0-470-64608-3|pages=504}}</ref>

=== Variation ===
The lobes of the lung are subject to [[anatomical variation]]s.<ref name="Moore2">{{cite book |last1=Moore |first1=K |title=Clinically oriented anatomy |date=2018 |isbn=978-1-4963-4721-3 |page=342 |publisher=Wolters Kluwer |edition=8th}}</ref> A horizontal interlobar fissure was found to be incomplete in 25% of right lungs, or even absent in 11% of all cases. An accessory fissure was also found in 14% and 22% of left and right lungs, respectively.<ref>{{Cite journal|date=2019-06-09|title=Variations in the lobes and fissures of lungs – a study in South Indian lung specimens|url=http://www.eurjanat.com/web/paper.php?id=120051lq|journal=European Journal of Anatomy|language=en|volume=18|issue=1|pages=16–20|issn=1136-4890}}</ref> An oblique fissure was found to be incomplete in 21% to 47% of left lungs.<ref>{{cite journal |last1=Meenakshi |first1=S |last2=Manjunath |first2=KY |last3=Balasubramanyam |first3=V |title=Morphological variations of the lung fissures and lobes. |journal=The Indian Journal of Chest Diseases & Allied Sciences |date=2004 |volume=46 |issue=3 |pages=179–82 |pmid=15553206 }}</ref> In some cases a fissure is absent, or extra, resulting in a right lung with only two lobes, or a left lung with three lobes.<ref name="Moore2"/>

A variation in the airway branching structure has been found specifically in the central airway
branching. This variation is associated with the development of [[COPD]] in adulthood.<ref>{{Cite journal|title=Human lung development:recent progress and new challenges|last=Marko|first=Z|journal=Development|volume=145|issue=16|article-number=dev163485|doi=10.1242/dev.163485|pmid=30111617|pmc=6124546|year=2018}}</ref>

== Development ==
{{Further|Sonic hedgehog#Lung development}}
The development of the human lungs arise from the [[laryngotracheal groove]] and develop to maturity over several weeks in the foetus and for several years following birth.<ref name="Sadler">{{cite book|last1=Sadler|first1=T.|title=Langman's medical embryology|url=https://archive.org/details/langmansmedicale00sadl_655|url-access=limited|date=2010|publisher=Lippincott Williams & Wilkins|location=Philadelphia|isbn=978-0-7817-9069-7|pages=[https://archive.org/details/langmansmedicale00sadl_655/page/n215 204]–207|edition=11th}}</ref>

The [[larynx]], [[trachea]], [[bronchus|bronchi]] and lungs that make up the respiratory tract, begin to form during the fourth week of [[human embryogenesis|embryogenesis]]<ref name=Moore_2002>{{cite book | author = Moore, K.L.|author2=Persaud, T.V.N. | title = The Developing Human: Clinically Oriented Embryology | edition = 7th| publisher = Saunders | year = 2002 | isbn = 978-0-7216-9412-2 }}</ref> from the [[lung bud]] which appears ventrally to the caudal portion of the [[foregut]].<ref>{{cite web|last1=Hill|first1=Mark|title=Respiratory System Development|url=https://embryology.med.unsw.edu.au/embryology/index.php/Respiratory_System_Development#cite_note-PMID20614584-22|website=UNSW Embryology|access-date=23 February 2016}}</ref>
[[File:Gray949.png|right|thumb|Lungs during development, showing the early branching of the primitive bronchial buds]]
The respiratory tract has a branching structure, and is also known as the respiratory tree.<ref name="Miura">{{Cite book|last1=Miura|first1=T|title=Multiscale Modeling of Developmental Systems|date=2008|volume=81|pages=291–310|doi=10.1016/S0070-2153(07)81010-6|pmid=18023732|series=Current Topics in Developmental Biology|isbn=978-0-12-374253-7|chapter=Modeling Lung Branching Morphogenesis}}</ref> In the embryo this structure is developed in the process of [[Morphogenesis#Branching morphogenesis|branching morphogenesis]],<ref name="Ochoa-Espinosa">{{cite journal |last1=Ochoa-Espinosa |first1=A |last2=Affolter |first2=M |title=Branching morphogenesis: from cells to organs and back. |journal=Cold Spring Harbor Perspectives in Biology |date=1 October 2012 |volume=4 |issue=10 |article-number=a008243 |doi=10.1101/cshperspect.a008243 |pmid=22798543|pmc=3475165 }}</ref> and is generated by the repeated splitting of the tip of the branch. In the development of the lungs (as in some other organs) the epithelium forms branching tubes. The lung has a left-right symmetry and each bud known as a [[lung bud|bronchial bud]] grows out as a tubular epithelium that becomes a bronchus. Each bronchus branches into bronchioles.<ref name="Wolpert">{{cite book|last1=Wolpert|first1=Lewis|title=Principles of development|date=2015|publisher=Oxford University Press|isbn=978-0-19-967814-3|pages=499–500|edition=5th}}</ref> The branching is a result of the tip of each tube bifurcating.<ref name="Miura"/> The branching process forms the bronchi, bronchioles, and ultimately the alveoli.<ref name="Miura"/> The four genes mostly associated with branching morphogenesis in the lung are the [[Hedgehog signaling pathway|intercellular signalling protein]] – [[sonic hedgehog]] (SHH), [[fibroblast growth factor]]s [[FGF10]] and FGFR2b, and [[bone morphogenetic protein]] [[bone morphogenetic protein 4|BMP4]]. FGF10 is seen to have the most prominent role. FGF10 is a [[paracrine signalling]] molecule needed for epithelial branching, and SHH inhibits FGF10.<ref name="Miura"/><ref name="Wolpert"/> The development of the alveoli is influenced by a different mechanism whereby continued bifurcation is stopped and the distal tips become dilated to form the alveoli.

At the end of the fourth week, the lung bud divides into two, the right and left [[lung bud|primary bronchial buds]] on each side of the trachea.<ref>{{cite book|last1=Sadler|first1=T.|title=Langman's medical embryology|url=https://archive.org/details/langmansmedicale00sadl_655|url-access=limited|date=2010|publisher=Lippincott Williams & Wilkins|location=Philadelphia|isbn=978-0-7817-9069-7|pages=[https://archive.org/details/langmansmedicale00sadl_655/page/n213 202]–204|edition=11th}}</ref><ref name="Larsen2001">{{cite book|last1=Larsen|first1=William J.|title=Human embryology|date=2001|publisher=Churchill Livingstone|location=Philadelphia|isbn=978-0-443-06583-5|page=144|edition=3.}}</ref> During the fifth week, the right bud branches into three secondary bronchial buds and the left branches into two secondary bronchial buds. These give rise to the lobes of the lungs, three on the right and two on the left. Over the following week, the secondary buds branch into tertiary buds, about ten on each side.<ref name=Larsen2001 /> From the sixth week to the sixteenth week, the major elements of the lungs appear except the [[Pulmonary alveolus|alveoli]].<ref name="isbn0-7817-5309-0">{{cite book |author=Kyung Won, Chung |title=Gross Anatomy (Board Review) |publisher=Lippincott Williams & Wilkins |location=Hagerstown, MD |year=2005 |page=156 |isbn=978-0-7817-5309-8 }}</ref> From week 16 to week 26, the bronchi enlarge and lung tissue becomes highly vascularised. Bronchioles and alveolar ducts also develop. By week 26, the terminal bronchioles have formed which branch into two respiratory bronchioles.<ref>{{cite book|last1=Larsen|first1=William J.|title=Human embryology|date=2001|publisher=Churchill Livingstone|location=Philadelphia|isbn=978-0-443-06583-5|page=134|edition=3.}}</ref> During the period covering the 26th week until birth the important [[blood–air barrier]] is established. Specialised [[alveolar cells|type I alveolar cells]] where [[gas exchange]] will take place, together with the [[alveolar cells|type II alveolar cells]] that secrete [[pulmonary surfactant]], appear. The surfactant reduces the [[surface tension]] at the air-alveolar surface which allows expansion of the alveolar sacs. The alveolar sacs contain the primitive alveoli that form at the end of the alveolar ducts,<ref>{{cite book|last1=Alberts|first1=Daniel|title=Dorland's illustrated medical dictionary|date=2012|publisher=Saunders/Elsevier|location=Philadelphia|isbn=978-1-4160-6257-8|page=56|edition=32nd}}</ref>
and their appearance around the seventh month marks the point at which limited respiration would be possible, and the premature baby could survive.<ref name=Sadler/>

=== Vitamin A deficiency ===
{{Main|Vitamin A deficiency}}
The developing lung is particularly vulnerable to changes in the levels of [[vitamin A]]. [[Vitamin A deficiency]] has been linked to changes in the epithelial lining of the lung and in the lung parenchyma. This can disrupt the normal physiology of the lung and predispose to respiratory diseases. Severe nutritional deficiency in vitamin A results in a reduction in the formation of the alveolar walls (septa) and to notable changes in the respiratory epithelium; alterations are noted in the extracellular matrix and in the protein content of the basement membrane. The extracellular matrix maintains lung elasticity; the basement membrane is associated with alveolar epithelium and is important in the blood-air barrier. The deficiency is associated with functional defects and disease states. Vitamin A is crucial in the development of the alveoli which continues for several years after birth.<ref name=Timoneda>{{cite journal |last1=Timoneda |first1=Joaquín |last2=Rodríguez-Fernández |first2=Lucía |last3=Zaragozá |first3=Rosa |last4=Marín |first4=M. |last5=Cabezuelo |first5=M. |last6=Torres |first6=Luis |last7=Viña |first7=Juan |last8=Barber |first8=Teresa |title=Vitamin A Deficiency and the Lung |journal=Nutrients |date=21 August 2018 |volume=10 |issue=9 |page=1132 |doi=10.3390/nu10091132 |pmid=30134568 |pmc=6164133 |doi-access=free }}</ref>

=== After birth ===
At [[Childbirth|birth]], the baby's lungs are filled with fluid secreted by the lungs and are not inflated. [[Adaptation to extrauterine life#Breathing and circulation|After birth]] the infant's [[central nervous system]] reacts to the sudden change in temperature and environment. This triggers the first breath, within about ten seconds after delivery.<ref name="Medline2015">{{cite web |title=Changes in the newborn at birth |url=https://medlineplus.gov/ency/article/002395.htm |website=MedlinePlus Medical Encyclopedia }}</ref> Before birth, the lungs are filled with fetal lung fluid.<ref>{{cite journal|title=Fetal lung liquid secretion|journal=American Journal of Respiratory Cell and Molecular Biology|volume=25|issue=1|pages=8–10|doi=10.1165/ajrcmb.25.1.f211|pmid=11472968|year=2001|last1=O'Brodovich|first1=Hugh}}</ref>      After the first breath, the fluid is quickly absorbed into the body or exhaled. The [[vascular resistance|resistance]] in the lung's blood vessels decreases giving an increased surface area for gas exchange, and the lungs begin to breathe spontaneously. This accompanies [[Adaptation to extrauterine life|other changes]] which result in an increased amount of blood entering the lung tissues.<ref name= Medline2015 />

At birth, the lungs are very undeveloped with only around one sixth of the alveoli of the adult lung present.<ref name="Sadler"/> The alveoli continue to form into early adulthood, and their ability to form when necessary is seen in the regeneration of the lung.<ref name="Schittny">{{cite journal|last1=Schittny|first1=JC|last2=Mund|first2=SI|last3=Stampanoni|first3=M|title=Evidence and structural mechanism for late lung alveolarization|journal=American Journal of Physiology. Lung Cellular and Molecular Physiology|date=February 2008|volume=294|issue=2|pages=L246–254|doi=10.1152/ajplung.00296.2007|pmid=18032698|citeseerx=10.1.1.420.7315}}</ref><ref name="SchittnyJC">{{cite journal|last1=Schittny|first1=JC|title=Development of the lung|journal=Cell and Tissue Research|date=March 2017|volume=367|issue=3|pages=427–444|doi=10.1007/s00441-016-2545-0|pmid=28144783|pmc=5320013}}</ref> Alveolar septa have a double [[Capillary#Structure|capillary network]] instead of the single network of the developed lung. Only after the maturation of the capillary network can the lung enter a normal phase of growth. Following the early growth in numbers of alveoli there is another stage of the alveoli being enlarged.<ref>{{cite journal|last1=Burri|first1=PH|title=Fetal and postnatal development of the lung|journal=Annual Review of Physiology|date=1984|volume=46|pages=617–628|pmid=6370120|doi=10.1146/annurev.ph.46.030184.003153}}</ref>

== 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 |page=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 /> 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 | archive-url=https://web.archive.org/web/20170404215608/https://www.nih.gov/news-events/nih-research-matters/overlooked-role-lungs-blood-formation | archive-date=April 4, 2017 | title=An overlooked role for lungs in blood formation| work=National Institutes of Health (NIH) | date=2017-04-03}}</ref>

== Gene and protein expression ==
{{Further |Bioinformatics#Gene and protein expression}}
About 20,000 [[Human genome#Coding sequences (protein-coding genes)|protein coding genes]] are expressed in human cells and almost 75% of these genes are expressed in the normal lung.<ref>{{Cite web|url=https://www.proteinatlas.org/humanproteome/lung|title=The human proteome in lung – The Human Protein Atlas|website=www.proteinatlas.org|access-date=2017-09-25}}</ref><ref>{{cite journal |display-authors=1 |last1=Uhlén |first1=Mathias |last2=Fagerberg |first2=Linn |last3=Hallström |first3=Björn M. |last4=Lindskog |first4=Cecilia |last5=Oksvold |first5=Per |last6=Mardinoglu |first6=Adil |last7=Sivertsson |first7=Åsa |last8=Kampf |first8=Caroline |last9=Sjöstedt |first9=Evelina |last10=Asplund |first10=Anna |last11=Olsson |first11=IngMarie |last12=Edlund |first12=Karolina |last13=Lundberg |first13=Emma |last14=Navani |first14=Sanjay |last15=Szigyarto |first15=Cristina Al-Khalili |last16=Odeberg |first16=Jacob |last17=Djureinovic |first17=Dijana |last18=Takanen |first18=Jenny Ottosson |last19=Hober |first19=Sophia |last20=Alm |first20=Tove |last21=Edqvist |first21=Per-Henrik |last22=Berling |first22=Holger |last23=Tegel |first23=Hanna |last24=Mulder |first24=Jan |last25=Rockberg |first25=Johan |last26=Nilsson |first26=Peter |last27=Schwenk |first27=Jochen M. |last28=Hamsten |first28=Marica |last29=Feilitzen |first29=Kalle von |last30=Forsberg |first30=Mattias |last31=Persson |first31=Lukas |last32=Johansson |first32=Fredric |last33=Zwahlen |first33=Martin |last34=Heijne |first34=Gunnar von |last35=Nielsen |first35=Jens |last36=Pontén |first36=Fredrik |title=Tissue-based map of the human proteome |journal=Science |date=23 January 2015 |volume=347 |issue=6220 |article-number=1260419 |doi=10.1126/science.1260419 |pmid=25613900 |citeseerx=10.1.1.665.2415 |s2cid=802377 }}</ref> A little less than 200 of these genes are more specifically expressed in the lung with less than 20 genes being highly lung specific. The highest expression of lung specific proteins are different [[surfactant]] proteins,<ref name=pmid26475269/> such as [[Surfactant protein A1|SFTPA1]], [[Surfactant protein B|SFTPB]] and [[Surfactant protein C|SFTPC]], and [[NAPSA|napsin]], expressed in type II pneumocytes. Other proteins with elevated expression in the lung are the [[dynein]] protein [[DNAH5]] in ciliated cells, and the secreted [[Uteroglobin|SCGB1A1]] protein in mucus-secreting  [[goblet cell]]s of the airway mucosa.<ref>{{cite journal |display-authors=1 |last1=Lindskog |first1=Cecilia |last2=Fagerberg |first2=Linn |last3=Hallström |first3=Björn |last4=Edlund |first4=Karolina |last5=Hellwig |first5=Birte |last6=Rahnenführer |first6=Jörg |last7=Kampf |first7=Caroline |last8=Uhlén |first8=Mathias |last9=Pontén |first9=Fredrik |last10=Micke |first10=Patrick |title=The lung-specific proteome defined by integration of transcriptomics and antibody-based profiling |journal=The FASEB Journal |date=28 August 2014 |volume=28 |issue=12 |pages=5184–5196 |doi=10.1096/fj.14-254862 |pmid=25169055 |doi-access=free }}</ref>

== Clinical significance ==
{{Main|Respiratory disease|Pulmonology}}
Lungs can be affected by a number of diseases and disorders. [[Pulmonology]] is the [[specialty (medicine)|medical speciality]] that deals with [[respiratory disease]]s involving the lungs and [[respiratory system]].<ref name="ACP">{{cite web|author1=American College of Physicians|title=Pulmonology|url=https://www.acponline.org/patients_families/about_internal_medicine/subspecialties/pulmonology/|publisher=ACP|access-date=9 February 2016|archive-url=https://web.archive.org/web/20150909203508/https://www.acponline.org/patients_families/about_internal_medicine/subspecialties/pulmonology/|archive-date=9 September 2015|author1-link=American College of Physicians}}</ref> [[Cardiothoracic surgery]] deals with [[surgery]] of the lungs including [[lung volume reduction surgery]], [[lobectomy]], [[pneumectomy]] and [[lung transplantation]].<ref>{{cite web|title=The Surgical Specialties: 8 – Cardiothoracic Surgery|url=https://www.rcseng.ac.uk/media/media-background-briefings-and-statistics/the-surgical-specialties-8-2013-cardiothoracic-surgery|website=Royal College of Surgeons|access-date=9 February 2016}}</ref>

===Inflammation and infection===
{{See also|Ghon focus}}
[[Inflammation|Inflammatory]] conditions of the lung tissue are [[pneumonia]], of the respiratory tract are [[bronchitis]] and [[bronchiolitis]], and of the [[pulmonary pleurae|pleurae]] surrounding the lungs [[pleurisy]]. Inflammation is usually caused by [[infection]]s due to [[bacteria]] or [[virus]]es. When the lung tissue is inflamed due to other causes it is called [[pneumonitis]]. One major cause of [[bacterial pneumonia]] is [[tuberculosis]].<ref name=DAVIDSONS2014 /> [[Chronic condition|Chronic]] infections often occur in those with [[immunodeficiency]] and can include a [[fungal infection]] by ''[[Aspergillus fumigatus]]'' that can lead to an [[aspergilloma]] forming in the lung.<ref name=DAVIDSONS2014 /><ref>{{cite web|title=Aspergilloma|website=Medical Dictionary|publisher=TheFreeDictionary|url=http://medical-dictionary.thefreedictionary.com/Aspergilloma}}</ref> In the US certain species of rat can transmit a [[hantavirus]] to humans that can cause untreatable [[hantavirus pulmonary syndrome]] with a similar presentation to that of [[acute respiratory distress syndrome]] (ARDS).<ref name="CDC1">{{cite web |title=Clinical Manifestation {{!}} Hantavirus {{!}} DHCPP {{!}} CDC |url=https://www.cdc.gov/hantavirus/technical/hps/clinical-manifestation.html |website=www.cdc.gov |access-date=7 January 2023 |language=en-us |date=21 February 2019}}</ref>

Alcohol affects the lungs and can cause inflammatory [[alcoholic lung disease]]. Acute exposure to alcohol stimulates the beating of [[cilia]] in the respiratory epithelium. However, chronic exposure has the effect of desensitising the ciliary response which reduces [[mucociliary clearance]] (MCC). MCC is an innate defense system protecting against pollutants and pathogens, and when this is disrupted the numbers of [[alveolar macrophage]]s are decreased. A subsequent inflammatory response is the release of [[cytokine]]s. Another consequence is the susceptibility to infection.<ref name="Arvers">{{cite journal |last1=Arvers |first1=P |title=[Alcohol consumption and lung damage: Dangerous relationships]. |journal=Revue des maladies respiratoires |date=December 2018 |volume=35 |issue=10 |pages=1039–1049 |doi=10.1016/j.rmr.2018.02.009 |pmid=29941207|s2cid=239523761 }}</ref><ref name="Slovinsky">{{cite journal |last1=Slovinsky |first1=WS |last2=Romero |first2=F |last3=Sales |first3=D |last4=Shaghaghi |first4=H |last5=Summer |first5=R |title=The involvement of GM-CSF deficiencies in parallel pathways of pulmonary alveolar proteinosis and the alcoholic lung. |journal=Alcohol (Fayetteville, N.Y.) |date=November 2019 |volume=80 |pages=73–79 |doi=10.1016/j.alcohol.2018.07.006 |pmid=31229291|pmc=6592783 }}</ref>

===Blood-supply changes===
[[File:InfarctPandLbasilarsegmentsPE.PNG|thumb|[[Infarction|Tissue death]] of the lung due to a [[pulmonary embolism]]]]
<!-- Blood supply -->A [[pulmonary embolism]] is a blood clot that becomes lodged in the [[pulmonary arteries]]. The majority of emboli arise because of [[deep vein thrombosis]] in the legs. Pulmonary emboli may be investigated using a [[ventilation/perfusion scan]], [[CT pulmonary angiography|a CT scan of the arteries of the lung]], or blood tests such as the [[D-dimer]].<ref name=DAVIDSONS2014 /> [[Pulmonary hypertension]] describes an increased pressure at the beginning of the [[pulmonary artery]] that has a large number of differing causes.<ref name=DAVIDSONS2014 /> Other rarer conditions may also affect the blood supply of the lung, such as [[granulomatosis with polyangiitis]], which causes inflammation of the small blood vessels of the lungs and kidneys.<ref name=DAVIDSONS2014 />

A [[pulmonary contusion|lung contusion]] is a bruise caused by chest trauma. It results in hemorrhage of the alveoli causing a build-up of fluid which can impair breathing, and this can be either mild or severe.
<!-- Intrapleural collections -->The function of the lungs can also be affected by compression from fluid in the pleural cavity [[pleural effusion]], or other substances such as air ([[pneumothorax]]), blood ([[hemothorax]]), or rarer causes. These may be investigated using a [[chest X-ray]] or [[CT scan]], and may require the insertion of a [[surgical drain]] until the underlying cause is identified and treated.<ref name=DAVIDSONS2014>{{cite book|editor1=Brian R. Walker|editor2=Nicki R. Colledge|editor3=Stuart H. Ralston|editor4=Ian D. Penman|others=Illustrations by Robert Britton|title=Davidson's principles and practice of medicine|date=2014|publisher=Churchill Livingstone/Elsevier |isbn=978-0-7020-5035-0|edition=22nd}}</ref>

===Obstructive lung diseases===
[[File: Respiratory tract.jpg|thumb|3D still image of constricted airways as in bronchial asthma]]
[[File:Emphysema H and E.jpg|thumb|Lung tissue affected by emphysema using [[H&E stain]]]]
<!-- Obstructive lung disease-->[[Asthma]], [[bronchiectasis]], and [[chronic obstructive pulmonary disease]] (COPD) that includes [[Bronchitis#Chronic bronchitis|chronic bronchitis]], and [[emphysema]], are all [[obstructive lung disease]]s characterised by [[airway obstruction]]. This limits the amount of air that is able to enter alveoli because of constriction of the bronchial tree, due to inflammation. Obstructive lung diseases are often identified because of symptoms and diagnosed with [[pulmonary function tests]] such as [[spirometry]]. 

Many obstructive lung diseases are managed by avoiding triggers (such as [[dust mite]]s or [[smoking]]), with symptom control such as [[bronchodilator]]s, and with suppression of inflammation (such as through [[corticosteroid]]s) in severe cases. A common cause of chronic bronchitis, and emphysema, is smoking; and common causes of [[bronchiectasis]] include severe infections and [[cystic fibrosis]]. The definitive cause of [[asthma]] is not yet known, but it has been linked to other atopic diseases.<ref name=DAVIDSONS2014 /><ref>{{Cite journal |last1=Galli |first1=Elena |last2=Gianni |first2=Simona |last3=Auricchio |first3=Giovanni |last4=Brunetti |first4=Ercole |last5=Mancino |first5=Giorgio |last6=Rossi |first6=Paolo |date=2007-09-01 |title=Atopic dermatitis and asthma |url=http://www.ingentaconnect.com/content/10.2500/aap2007.28.3048 |journal=Allergy and Asthma Proceedings |language=en |volume=28 |issue=5 |pages=540–543 |doi=10.2500/aap2007.28.3048 |pmid=18034972 |issn=1088-5412}}</ref>

The breakdown of alveolar tissue, often as a result of tobacco-smoking leads to emphysema, which can become severe enough to develop into COPD. [[Elastase]] breaks down the [[elastin]] in the lung's connective tissue that can also result in emphysema. Elastase is inhibited by the [[acute-phase protein]], [[alpha-1 antitrypsin]], and when there is a [[Alpha-1 antitrypsin deficiency|deficiency]] in this, emphysema can develop. With persistent stress from smoking, the [[airway basal cell]]s become disarranged and lose their regenerative ability needed to repair the epithelial barrier. The disorganised basal cells are seen to be responsible for the major airway changes that are characteristic of [[COPD]], and with continued stress can undergo a malignant transformation. Studies have shown that the initial development of emphysema is centred on the early changes in the airway epithelium of the small airways.<ref name="Crystal" /> Basal cells become further deranged in a smoker's transition to clinically defined COPD.<ref name="Crystal">{{cite journal |last1=Crystal |first1=RG |title=Airway basal cells. The "smoking gun" of chronic obstructive pulmonary disease. |journal=American Journal of Respiratory and Critical Care Medicine |date=15 December 2014 |volume=190 |issue=12 |pages=1355–62 |doi=10.1164/rccm.201408-1492PP |pmid=25354273|pmc=4299651 }}</ref>

===Restrictive lung diseases===
<!-- Restrictive lung disease-->Some types of chronic lung diseases are classified as [[restrictive lung disease]], because of a restriction in the amount of lung tissue involved in respiration. These include [[pulmonary fibrosis]] which can occur when the lung is inflamed for a long period of time. [[Fibrosis]] in the lung replaces functioning lung tissue with [[fibrous connective tissue]]. This can be due to a large variety of [[occupational lung disease]]s such as [[Coalworker's pneumoconiosis]], [[autoimmune disease]]s or more rarely to a reaction to [[medication]].<ref name=DAVIDSONS2014 /> Severe respiratory disorders, where spontaneous breathing is not enough to maintain life, may need the use of [[mechanical ventilation]] to ensure an adequate supply of air.

===Cancers===
<!-- Cancer-->[[Lung cancer]] can either arise directly from lung tissue or as a result of [[metastasis]] from another part of the body. There are two main types of primary tumour described as either [[small-cell lung carcinoma|small-cell]] or [[non-small-cell lung carcinoma]]s. The major risk factor for cancer is [[tobacco smoking|smoking]]. Once a cancer is identified it is [[tumour staging|staged]] using scans such as a [[CT scan]] and a sample of tissue from a [[biopsy]] is taken. Cancers may be treated surgically by removing the tumour, the use of [[radiotherapy]], [[chemotherapy]] or a combination, or with the aim of [[palliative care|symptom control]].<ref name=DAVIDSONS2014 /> [[Lung cancer screening]] is being recommended in the United States for high-risk populations.<ref>{{cite web |url=http://www.uspreventiveservicestaskforce.org/uspstf/uspslung.htm |title=Lung Cancer Screening |year=2013 |publisher=[[United States Preventive Services Task Force|U.S. Preventative Services Task Force]] |access-date=2016-07-10 |archive-url=https://web.archive.org/web/20101104055403/http://www.uspreventiveservicestaskforce.org/uspstf/uspslung.htm |archive-date=2010-11-04 }}</ref>

===Congenital disorders===
[[Congenital disorder]]s include [[cystic fibrosis]], [[pulmonary hypoplasia]] (an incomplete development of the lungs)<ref name=Cadichon>{{citation
|last=Cadichon
|first=Sandra B.
|contribution=Chapter 22: Pulmonary hypoplasia
|title=Congenital malformations: evidence-based evaluation and management
|editor1-last= Kumar
|editor1-first=Praveen
|editor2-last=Burton
|editor2-first=Barbara K.
|year=2007
|contribution-url=https://books.google.com/books?id=n2M9tfQQN9cC&pg=PA143}}</ref>[[congenital diaphragmatic hernia]], and [[infant respiratory distress syndrome]] caused by a deficiency in lung surfactant. An [[azygos lobe]] is a congenital [[anatomical variation]] which though usually without effect can cause problems in [[thoracoscopy|thoracoscopic]] procedures.<ref>{{cite journal|last1=Sieunarine|first1=K.|last2=May|first2=J.|last3=White|first3=G.H.|last4=Harris|first4=J.P.|title=Anomalous azygos vein: a potential danger during endoscopic thoracic sypathectomy|journal=ANZ Journal of Surgery|date=August 1997|volume=67|issue=8|pages=578–579|doi=10.1111/j.1445-2197.1997.tb02046.x|pmid=9287933}}</ref>

===Pleural space pressure===
A [[pneumothorax]] (collapsed lung) is an abnormal collection of air in the [[pleural space]] that causes an uncoupling of the lung from the [[chest wall]].<ref name="BMJ2014">{{Cite journal |last1=Bintcliffe |first1=Oliver |last2=Maskell |first2=Nick |date=8 May 2014 |title=Spontaneous pneumothorax |journal=[[BMJ]] |volume=348 |article-number=g2928 |doi=10.1136/bmj.g2928 |pmid=24812003|s2cid=32575512 |url=http://www.bmj.com/content/bmj/348/bmj.g3302.full.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.bmj.com/content/bmj/348/bmj.g3302.full.pdf |archive-date=2022-10-09 |url-status=live }}</ref> The lung cannot expand against the air pressure inside the pleural space. An easy to understand example is a traumatic pneumothorax, where air enters the pleural space from outside the body, as occurs with puncture to the chest wall. Similarly, [[scuba diver]]s ascending while holding their breath with their lungs fully inflated can cause air sacs ([[Pulmonary alveolus|alveoli]]) to burst and leak high pressure air into the pleural space.

===Examination===
{{Main|Respiratory examination|Respiratory sounds}}
As part of a [[physical examination]] in response to respiratory symptoms of [[dyspnea|shortness of breath]], and [[cough]], a [[respiratory examination|lung examination]] may be carried out. This exam includes [[palpation]] and [[auscultation]].<ref>{{Cite book|title=Principles of Pulmonary Pathology|last1=Weinberger|first1=Steven|last2=Cockrill|first2=Barbara|last3=Mandell|first3=J|year=2019|isbn=978-0-323-52371-4|pages=30|publisher=Elsevier }}</ref> The areas of the lungs that can be [[auscultation|listened to]] using a stethoscope are called the [[lung fields]], and these are the posterior, lateral, and anterior lung fields. The posterior fields can be listened to from the back and include: the lower lobes (taking up three quarters of the posterior fields); the anterior fields taking up the other quarter; and the lateral fields under the [[axilla]]e, the left axilla for the lingual, the right axilla for the middle right lobe. The anterior fields can also be auscultated from the front.<ref>{{Cite web|url=https://meded.ucsd.edu/clinicalmed/lung.htm|title=Lung examination|website=meded.ucsd.edu|access-date=31 August 2019}}</ref> An area known as the [[triangle of auscultation]] is an area of thinner musculature on the back which allows improved listening.<ref name="Malik">{{cite book|last1=Malik |first1=N |last2=Tedder |first2=BL |last3=Zemaitis |first3=MR |title=Anatomy, Thorax, Triangle of Auscultation |date=January 2021 |pmid=30969656}}</ref> Abnormal [[respiratory sounds|breathing sounds]] heard during a lung exam can indicate the presence of a lung condition; [[wheezing]] for example is commonly associated with [[asthma]] and [[COPD]].

=== Function testing ===
{{Main|Pulmonary function testing|Lung volumes}}
{{multiple image
| total_width       = 600
| align             = right
| direction         = horizontal
| image1            = LungVolume.jpg
| caption1          = [[Lung volumes]] as described in the text
| image2            = DoingSpirometry.JPG
| caption2          = A person doing a [[spirometry]] test
}}

[[Pulmonary function testing|Lung function testing]] is carried out by evaluating a person's capacity to inhale and exhale in different circumstances.<ref name="Ganong2012_34">{{cite book|last1=Kim E.|first1=Barrett|title=Ganong's review of medical physiology|date=2012|publisher=McGraw-Hill Medical|location=New York|isbn=978-0-07-178003-2|chapter=Chapter 34. Introduction to Pulmonary Structure and Mechanics|edition=24th}}</ref> The volume of air inhaled and exhaled by a person at rest is the [[tidal volume]] (normally 500–750&nbsp;mL); the [[inspiratory reserve volume]] and [[expiratory reserve volume]] are the additional amounts a person is able to forcibly inhale and exhale respectively. The summed total of forced inspiration and expiration is a person's [[vital capacity]]. Not all air is expelled from the lungs even after a forced breath out; the remainder of the air is called the [[Lung volumes|residual volume]]. Together these terms are referred to as [[lung volumes]].<ref name="Ganong2012_34" />

Pulmonary [[plethysmograph]]s are used to measure [[functional residual capacity]].<ref>{{cite journal|last1=Criée|first1=C.P.|last2=Sorichter|first2=S.|last3=Smith|first3=H.J.|last4=Kardos|first4=P.|last5=Merget|first5=R.|last6=Heise|first6=D.|last7=Berdel|first7=D.|last8=Köhler|first8=D.|last9=Magnussen|first9=H.|last10=Marek|first10=W.|last11=Mitfessel|first11=H.|last12=Rasche|first12=K.|last13=Rolke|first13=M.|last14=Worth|first14=H.|last15=Jörres|first15=R.A.|title=Body plethysmography – Its principles and clinical use|journal=Respiratory Medicine|date=July 2011|volume=105|issue=7|pages=959–971|doi=10.1016/j.rmed.2011.02.006|pmid=21356587|doi-access=free}}</ref> Functional residual capacity cannot be measured by tests that rely on breathing out, as a person is only able to breathe a maximum of 80% of their total functional capacity.<ref name=Applegate2014 /> The total lung capacity depends on the person's age, height, weight, and sex, and normally ranges between four and six litres.<ref name=Ganong2012_34 /> Females tend to have a 20–25% lower capacity than males. Tall people tend to have a larger total lung capacity than shorter people. [[tobacco smoking|Smokers]] have a lower capacity than nonsmokers. Thinner persons tend to have a larger capacity. Lung capacity can be increased by physical training as much as 40% but the effect may be modified by exposure to air pollution.<ref name=Applegate2014>{{cite book|last1=Applegate|first1=Edith|title=The Anatomy and Physiology Learning System|date=2014|publisher=Elsevier Health Sciences|isbn=978-0-323-29082-1|page=335|url=https://books.google.com/books?id=c8nsAwAAQBAJ&q=factors+that+influence+total+lung+capacity&pg=PA335}}</ref><ref name="Laeremans et al. 2018">{{cite journal  | last = Laeremans  | first = M  | title = Black Carbon Reduces the Beneficial Effect of Physical Activity on Lung Function  | journal = Medicine and Science in Sports and Exercise  | volume =   50| issue = 9  | pages = 1875–1881  | year = 2018  | doi = 10.1249/MSS.0000000000001632| pmid = 29634643  | hdl = 10044/1/63478 | s2cid = 207183760 | hdl-access = free  }}</ref>

Other lung function tests include [[spirometry]], measuring the amount (volume) and flow of air that can be inhaled and exhaled. The maximum volume of breath that can be exhaled is called the [[vital capacity]]. In particular, how much a person is able to exhale in one second (called [[Spirometry#Parameters|forced expiratory volume]] (FEV1)) as a proportion of how much they are able to exhale in total (FEV). This ratio, the FEV1/FEV ratio, is important to distinguish whether a lung disease is [[restrictive lung disease|restrictive]] or [[obstructive lung disease|obstructive]].<ref name=DAVIDSONS2014 /><ref name=Ganong2012_34 /> Another test is that of the lung's  [[diffusing capacity]] – this is a measure of the transfer of gas from air to the blood in the lung capillaries.

== Culinary uses ==
[[File:Öpke-hésip at Ancient City of Kashi (20230923195242).jpg|thumb|upright=0.8|''Öpke-hésip'', a Uyghur dish made with [[mutton|lamb]] lung and [[rice]] sausage]]
Mammal lung is one of the main types of [[offal]], or pluck, alongside the [[heart]] and [[trachea]], and is consumed as a foodstuff around the world in dishes such as Scottish [[haggis]]. The United States [[Food and Drug Administration]] legally prohibits the sale of animal lungs due to concerns such as [[fungal spore]]s or cross-contamination with other organs, although this has been criticised as unfounded.<ref name=IllegalLungs>Davies, Madeline. [https://www.eater.com/22774656/gastropod-offal-episode-edible-organs-lungs-testicles-brains "Here's Why It's Illegal to Sell Animal Lungs for Consumption in the U.S."], ''[[Eater (website)|Eater]]'', 10 November 2021. Retrieved 26 January 2023.</ref>

== Other animals ==
=== Birds ===
{{Main|Bird anatomy#Respiratory system}}
[[File:BirdRespiration.svg|thumb|left|On inhalation, air travels to air sacs near the back of a bird. The air then passes through the lungs to air sacs near the front of the bird, from where the air is exhaled.]] [[File:Cross-current exchanger.jpg|thumb|right|The cross-current respiratory gas exchanger in the lungs of birds. Air is forced from the air sacs unidirectionally (from left to right in the diagram) through the parabronchi. The pulmonary capillaries surround the parabronchi in the manner shown (blood flowing from below the parabronchus to above it in the diagram).<ref name=AvResp>{{cite web| url = http://www.people.eku.edu/ritchisong/birdrespiration.html | title = BIO 554/754 – Ornithology: Avian respiration | access-date = 2009-04-23 | last = Ritchson | first = G | publisher = Department of Biological Sciences, Eastern Kentucky University }}</ref><ref name= graham>{{cite journal|last=Scott|first=Graham R.|title=Commentary: Elevated performance: the unique physiology of birds that fly at high altitudes|journal=Journal of Experimental Biology|volume= 214|issue=15|pages=2455–2462|date=2011|doi=10.1242/jeb.052548|pmid=21753038|doi-access=free}}</ref> Blood or air with a high oxygen content is shown in red; oxygen-poor air or blood is shown in various shades of purple-blue.]]
The lungs of birds are relatively small, but are connected to eight or nine [[air sacs]] that extend through much of the body, and are in turn connected to air spaces within the bones. On inhalation, air travels through the trachea of a bird into the air sacs. Air then travels continuously from the air sacs at the back, through the lungs, which are relatively fixed in size, to the air sacs at the front. From here, the air is exhaled. These fixed size lungs are called "circulatory lungs", as distinct from the "bellows-type lungs" found in most other animals.<ref name="AvResp"/><ref name="Maina2005">{{cite book|last1=Maina|first1=John N.|title=The lung air sac system of birds development, structure, and function; with 6 tables|date=2005|publisher=Springer|location=Berlin|isbn=978-3-540-25595-6|pages=3.2–3.3 "Lung", "Airway (Bronchiol) System" 66–82|url=https://books.google.com/books?id=-wtoEg7fcjkC&q=neopulmonic+parabronchi&pg=PA66}}</ref>

The lungs of birds contain millions of tiny parallel passages called [[parabronchi]]. Small sacs called ''atria'' radiate from the walls of the tiny passages; these, like the alveoli in other lungs, are the site of [[gas exchange]] by simple diffusion.<ref name=Maina2005 /> The blood flow around the parabronchi and their atria forms a cross-current process of gas exchange (see diagram on the right).<ref name=AvResp /><ref name= graham />

The air sacs, which hold air, do not contribute much to gas exchange, despite being thin-walled, as they are poorly vascularised. The air sacs expand and contract due to changes in the volume in the thorax and abdomen. This volume change is caused by the movement of the sternum and ribs and this movement is often synchronised with movement of the flight muscles.<ref name=VB>{{Cite book|last1=Romer |first1=Alfred Sherwood |last2=Parsons |first2=Thomas S. |year=1977 |title=The Vertebrate Body |publisher=Holt-Saunders International |location=Philadelphia |pages=330–334 |isbn=978-0-03-910284-5}}</ref>

Parabronchi in which the air flow is unidirectional are called ''paleopulmonic parabronchi'' and are found in all birds. Some birds, however, have, in addition, a lung structure where the air flow in the parabronchi is bidirectional. These are termed ''neopulmonic parabronchi''.<ref name=Maina2005 />

=== Reptiles ===
{{Main|Reptile anatomy#Respiratory system}}
The lungs of most reptiles have a single bronchus running down the centre, from which numerous branches reach out to individual pockets throughout the lungs. These pockets are similar to alveoli in mammals, but much larger and fewer in number. These give the lung a sponge-like texture. In [[tuatara]]s, [[snake]]s, and some [[lizard]]s, the lungs are simpler in structure, similar to that of typical amphibians.<ref name=VB />

Snakes and limbless lizards typically possess only the right lung as a major respiratory organ; the left lung is greatly reduced, or even absent. [[Amphisbaenian]]s, however, have the opposite arrangement, with a major left lung, and a reduced or absent right lung.<ref name=VB />

Both [[Crocodylia|crocodilians]] and [[Monitor Lizard|monitor lizards]] have lungs similar to those of birds, providing a unidirectional airflow and even possessing air sacs.<ref>{{cite web|title=Unidirectional airflow in the lungs of birds, crocs…and now monitor lizards!?|url=http://svpow.com/2013/12/11/unidirectional-airflow-in-the-lungs-of-birds-crocs-and-now-monitor-lizards/|website=Sauropod Vertebra picture of the week|access-date=9 February 2016|date=2013-12-11}}</ref> The now extinct [[Pterosauria|pterosaurs]] have seemingly even further refined this type of lung, extending the airsacs into the wing membranes and, in the case of [[Lonchodectidae|lonchodectids]], ''[[Tupuxuara]]'', and [[Azhdarchoidea|azhdarchoids]], the hindlimbs.<ref>{{cite journal|last1=Claessens|first1=Leon P.A.M.|last2=O'Connor|first2=Patrick M.|last3=Unwin|first3=David M.|last4=Sereno|first4=Paul|title=Respiratory Evolution Facilitated the Origin of Pterosaur Flight and Aerial Gigantism|journal=PLOS ONE|date=18 February 2009|volume=4|issue=2|article-number=e4497|doi=10.1371/journal.pone.0004497|pmid=19223979|pmc=2637988|bibcode=2009PLoSO...4.4497C|doi-access=free}}</ref>

[[Reptile|Reptilian]] lungs typically receive air via expansion and contraction of the ribs driven by [[axial skeleton|axial muscles]] and buccal pumping. [[Crocodilian]]s also rely on the [[hepatic]] piston method, in which the liver is pulled back by a muscle anchored to the [[pubis (bone)|pubic bone]] (part of the pelvis) called the diaphragmaticus,<ref name="Munns">{{cite journal|last1=Munns |first1=SL |last2=Owerkowicz |first2=T |last3=Andrewartha |first3=SJ |last4=Frappell |first4=PB |title=The accessory role of the diaphragmaticus muscle in lung ventilation in the estuarine crocodile Crocodylus porosus|journal=The Journal of Experimental Biology|date=1 March 2012|volume=215|issue=Pt 5|pages=845–852|pmid=22323207|doi=10.1242/jeb.061952|bibcode=2012JExpB.215..845M |doi-access=free}}</ref> which in turn creates negative pressure in the crocodile's thoracic cavity, allowing air to be moved into the lungs by [[Boyle's law]]. [[Turtle]]s, which are unable to move their ribs, instead use their forelimbs and [[pectoral girdle]] to force air in and out of the lungs.<ref name=VB />

=== Amphibians ===
{{Further|Frog#Respiration and circulation}}
[[File:Axolotl ganz.jpg|thumb|right|alt=Axolotl|The [[axolotl]] (''Ambystoma mexicanum'') retains its larval form with gills into adulthood.]]
The lungs of most [[frog]]s and other [[amphibian]]s are simple and balloon-like, with gas exchange limited to the outer surface of the lung. This is not very efficient, but amphibians have low metabolic demands and can also quickly dispose of carbon dioxide by diffusion across their skin in water, and supplement their oxygen supply by the same method. Amphibians employ a [[positive pressure]] system to get air to their lungs, forcing air down into the lungs by [[buccal pumping]]. This is distinct from most higher vertebrates, who use a breathing system driven by [[negative room pressure|negative pressure]] where the lungs are inflated by expanding the rib cage.<ref name=Breathing>{{cite journal |last1=Janis |first1=Christine M. |last2=Keller |first2=Julia C. |title=Modes of ventilation in early tetrapods: Costal aspiration as a key feature of amniotes  |journal=Acta Palaeontologica Polonica |date=2001 |volume=46 |issue=2 |pages=137–170 |url=https://www.app.pan.pl/article/item/app46-137.html }}</ref> In buccal pumping, the floor of the mouth is lowered, filling the mouth cavity with air. The throat muscles then presses the throat against the underside of the [[skull]], forcing the air into the lungs.<ref name="review">{{cite journal |last1=Brainerd |first1=E. L. |title=New perspectives on the evolution of lung ventilation mechanisms in vertebrates |journal=Experimental Biology Online |date=December 1999 |volume=4 |issue=2 |pages=1–28 |doi=10.1007/s00898-999-0002-1 |bibcode=1999EvBO....4b...1B |s2cid=35368264 }}</ref>

Due to the possibility of respiration across the skin combined with small size, all known lungless [[tetrapod]]s are amphibians. The majority of salamander species are [[lungless salamander]]s, which respirate through their skin and tissues lining their mouth. This necessarily restricts their size: all are small and rather thread-like in appearance, maximising skin surface relative to body volume.<ref name=Duellman&Trueb>{{cite book|last1=Duellman|first1=W.E.|last2=Trueb|first2=L. |others=illustrated by L. Trueb |title=Biology of amphibians|year=1994|publisher=Johns Hopkins University Press|isbn=978-0-8018-4780-6}}</ref> Other known lungless tetrapods are the [[Bornean flat-headed frog]]<ref>{{cite news |last1=Bickford |first1=David |title=First Lungless Frog Discovered in Indonesia |url=https://www.scientificamerican.com/gallery/first-lungless-frog-discovered-in-indonesia/ |work=Scientific American |date=April 15, 2008 }}</ref> and ''[[Atretochoana eiselti]]'', a [[caecilian]].<ref>{{cite journal |last1=Wilkinson |first1=M. |last2=Sebben |first2=A. |last3=Schwartz |first3=E.N.F. |last4=Schwartz |first4=C.A. |title=The largest lungless tetrapod: report on a second specimen of (Amphibia: Gymnophiona: Typhlonectidae) from Brazil |journal=Journal of Natural History |date=April 1998 |volume=32 |issue=4 |pages=617–627 |doi=10.1080/00222939800770321 }}</ref>

The lungs of amphibians typically have a few narrow internal walls ([[:wikt:septum|septa]]) of soft tissue around the outer walls, increasing the respiratory surface area and giving the lung a honeycomb appearance. In some salamanders, even these are lacking, and the lung has a smooth wall. In caecilians, as in snakes, only the right lung attains any size or development.<ref name=VB />

===Fish===
Lungs are found in three groups of fish; the [[coelacanth]]s, the [[bichir]]s and the [[lungfish]]. Like in tetrapods, but unlike fish with swim bladder, the opening is at the ventral side of the oesophagus. The coelacanth has a nonfunctional and unpaired vestigial lung surrounded by a fatty organ.<ref>{{cite journal | pmc=5717702 | date=2017 | last1=Lambertz | first1=M. | title=The vestigial lung of the coelacanth and its implications for understanding pulmonary diversity among vertebrates: New perspectives and open questions | journal=Royal Society Open Science | volume=4 | issue=11 | doi=10.1098/rsos.171518 | pmid=29291127 | bibcode=2017RSOS....471518L }}</ref> Bichirs, the only group of [[Actinopterygii|ray-finned fish]] with lungs, have a pair which are hollow unchambered sacs, where the gas-exchange occurs on very flat folds that increase their inner surface area. The lungs of [[lungfish]] show more resemblance to tetrapod lungs. There is an elaborate network of parenchymal septa, dividing them into numerous respiration chambers.<ref>{{cite book | url=https://books.google.com/books?id=3bsgS125KH0C&dq=Lungfish+complex+respiratory+surface+septa+honeycomb&pg=PA1864 | title=Encyclopedia of Fish Physiology: From Genome to Environment | date=June 2011 | publisher=Academic Press | isbn=978-0-08-092323-9 }}</ref><ref>{{cite journal | url=https://anatomypubs.onlinelibrary.wiley.com/doi/full/10.1002/ar.20576 | doi=10.1002/ar.20576 | title=Innervation and Neurotransmitter Localization in the Lung of the Nile bichir ''Polypterus bichir bichir'' | date=2007 | last1=Zaccone | first1=Giacomo | last2=Mauceri | first2=Angela | last3=Maisano | first3=Maria | last4=Giannetto | first4=Alessia | last5=Parrino | first5=Vincenzo | last6=Fasulo | first6=Salvatore | journal=The Anatomical Record | volume=290 | issue=9 | pages=1166–1177 | pmid=17722050 }}</ref> In the [[Australian lungfish]], there is only a single lung, albeit divided into two lobes. Other lungfish, however, have traditionally been considered having two lungs, but newer research defines paired lungs as bilateral lung buds that arise simultaneously and are both connected directly to the foregut, which is only seen in tetrapods.<ref>Camila Cupello, Tatsuya Hirasawa, Norifumi Tatsumi, Yoshitaka Yabumoto, Pierre Gueriau, Sumio Isogai, Ryoko Matsumoto, Toshiro Saruwatari, Andrew King, Masato Hoshino, Kentaro Uesugi, Masataka Okabe, Paulo M Brito (2022) [https://elifesciences.org/articles/77156 Lung evolution in vertebrates and the water-to-land transition], ''[[eLife]]''</ref> In all lungfish, including the Australian, the lungs are located in the upper dorsal part of the body, with the connecting duct curving around and above the oesophagus. The blood supply also twists around the oesophagus, suggesting that the lungs originally evolved in the ventral part of the body, as in other vertebrates.<ref name=VB />

=== Invertebrates ===
{{further|Respiratory system of gastropods}}
[[File:Spider internal anatomy-en.svg|thumb|[[Book lung]]s of a female spider (shown in pink)]]
A number of [[invertebrate]]s have lung-like structures that serve a similar respiratory purpose to true vertebrate lungs, but are not evolutionarily related and only arise out of [[convergent evolution]]. Some [[arachnid]]s, such as [[spider]]s and [[scorpion]]s, have structures called [[book lung]]s used for atmospheric gas exchange. Some species of spider have four pairs of book lungs but most have two pairs.<ref>{{Cite web
| url = https://www.britannica.com/science/book-lung
| title = book lung {{!}} anatomy
| website = Encyclopædia Britannica
| access-date = 2016-02-24
}}</ref> Scorpions have [[Spiracle (arthropods)|spiracle]]s on their body for the entrance of air to the book lungs.<ref>{{Cite web
| url = https://www.britannica.com/science/spiracle
| title = spiracle {{!}} anatomy
| website = Encyclopædia Britannica
| access-date = 2016-02-24
}}</ref>

The [[coconut crab]] is terrestrial and uses structures called [[branchiostegal lung]]s to breathe air.<ref name="Farrelly2005">{{cite journal|vauthors=Farrelly CA, Greenaway P|year=2005|title=The morphology and vasculature of the respiratory organs of terrestrial hermit crabs (''Coenobita'' and ''Birgus''): gills, branchiostegal lungs and abdominal lungs |journal=Arthropod Structure & Development|volume=34 |issue=1 |pages=63–87 |doi=10.1016/j.asd.2004.11.002|bibcode=2005ArtSD..34...63F }}</ref> Juveniles are released into the ocean, however adults cannot swim and possess an only rudimentary set of gills. The adult crabs can breathe on land and hold their breath underwater.<ref>{{Cite book
| url = https://books.google.com/books?id=RR09AAAAIAAJ
| title = Biology of the Land Crabs
| last1 = Burggren
| first1 = Warren W.
| last2 = McMahon
| first2 = Brian R.
| year = 1988
| publisher = Cambridge University Press
| isbn = 978-0-521-30690-4
| page = 25
| language = en
}}</ref> The branchiostegal lungs are seen as a developmental adaptive stage from water-living to enable land-living, or from fish to amphibian.<ref>{{Cite book
| url = https://books.google.com/books?id=RR09AAAAIAAJ
| title = Biology of the Land Crabs
| last1 = Burggren
| first1 = Warren W.
| last2 = McMahon
| first2 = Brian R.
| year = 1988
| publisher = Cambridge University Press
| isbn = 978-0-521-30690-4
| page = 331
| language = en
}}</ref>

[[Pulmonates]] are mostly [[land snail]]s and [[slug]]s that have developed a simple lung from the [[mantle cavity]]. An externally located opening called the [[pneumostome]] allows air to be taken into the mantle cavity lung.<ref>Land Snails (& other Air-Breathers in Pulmonata Subclass & Sorbeconcha Clade).
at Washington State University Tri-Cities Natural History Museum. Accessed 25 February 2016. http://shells.tricity.wsu.edu/ArcherdShellCollection/Gastropoda/Pulmonates.html {{Webarchive|url=https://web.archive.org/web/20181109010506/http://shells.tricity.wsu.edu/ArcherdShellCollection/Gastropoda/Pulmonates.html |date=2018-11-09 }}</ref><ref>{{Cite book
| url = https://books.google.com/books?id=7UyeBQAAQBAJ
| title = Mollusca: Metabolic Biochemistry and Molecular Biomechanics
| last = Hochachka
| first = Peter W.
| year = 2014
| publisher = Academic Press
| isbn = 978-1-4832-7603-8
| language = en
}}</ref>

== Evolutionary origins ==
The lungs of today's terrestrial [[vertebrate]]s and the [[gas bladder]]s of today's [[fish]] are believed to have evolved from simple sacs, as outpocketings of the [[esophagus|oesophagus]], that allowed early fish to gulp air under oxygen-poor conditions.<ref name=farmer>{{Cite journal |title=Did lungs and the intracardiac shunt evolve to oxygenate the heart in vertebrates |author=Colleen Farmer |journal=Paleobiology |year=1997 |url=http://www.biology.utah.edu/farmer/publications%20pdf/1997%20Paleobiology23.pdf |doi=10.1017/S0094837300019734 |volume=23 |pages=358–372 |issue=3 |bibcode=1997Pbio...23..358F |s2cid=87285937 |archive-url=https://web.archive.org/web/20100611170942/http://www.biology.utah.edu/farmer/publications%20pdf/1997%20Paleobiology23.pdf |archive-date=2010-06-11 }}</ref> These outpocketings first arose in the  [[Osteichthyes|bony fish]]. In most of the [[Actinopterygii|ray-finned fish]], the sacs evolved into closed off gas bladders, while a number of [[carp]], [[trout]], [[herring]], [[catfish]], and [[eel]]s have retained the [[physostome]] condition with the sac being open to the oesophagus. In more basal bony fish, such as the [[gar]], [[bichir]], [[bowfin]] and the [[Sarcopterygii|lobe-finned fish]], the sacs have evolved to primarily function as lungs.<ref name="farmer" /> The lobe-finned fish gave rise to the land-based [[tetrapod]]s. Thus, the lungs of vertebrates are [[homology (biology)|homologous]] to the gas bladders of fish (but not to their [[gill]]s).<ref>{{cite journal|last1=Longo|first1=Sarah|last2=Riccio|first2=Mark|last3=McCune|first3=Amy R|author3-link=Amy McCune|title=Homology of lungs and gas bladders: Insights from arterial vasculature|journal=Journal of Morphology|date=June 2013|volume=274|issue=6|pages=687–703|doi=10.1002/jmor.20128|pmid=23378277|bibcode=2013JMorp.274..687L |s2cid=29995935}}</ref>

== See also ==
{{col div|colwidth=20em}}
* [[Atelectasis]]
* [[Drowning]]
* [[Interstitial lung disease]]
* [[Liquid breathing]]
* [[Lung abscess]]
* [[Lung-on-a-chip]]
* [[Secarecytosis]]
* [[List of terms of lung size and activity]]
* [[Tobacco]]
* [[Vaping]]
* [[Smoking]]
{{colend}}

== References ==
{{Reflist}}

== External links ==
{{Commons category|Lungs|lcfirst=yes}}
* [https://www.proteinatlas.org/humanproteome/lung Lung at the Human Protein Atlas]
{{Human systems and organs }}
{{Lower respiratory system anatomy|state=expanded}}
{{Respiratory physiology}}
{{Authority control}}

{{DEFAULTSORT:Human Lung}}
[[Category:Lung| ]]
[[Category:Human anatomy by organ]]
[[Category:Respiratory system anatomy]]
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