Editing Lung (section)

== 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 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|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|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=9780071222075 |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=9780120342709 }}</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 | page = 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=9780128104897 |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=9780323523714 |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=9780323523714|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 |pages=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|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|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 |pages=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 |pages=120 |isbn=978-0-8247-0401-8 |url=https://books.google.com/books?id=pAuiWvNHwZcC&q=area+tennis+court+alveoli&pg=PA120|access-date=2008-10-11}}</ref>

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

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

===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=9781496347213 |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=9781416045748|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=9780470646083|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=9781496347213 |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|pages=dev163485|doi=10.1242/dev.163485|pmid=30111617|pmc=6124546|year=2018}}</ref>