Editing Lung (section)

=== 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>