Editing Pulmonary alveolus (section)

== Function ==
[[File:An annotated diagram of an alveolus.svg|thumb|An annotated diagram of the alveolus]]

===Type I cells===
[[File:Cross section of an alveolus and capillaries showing diffusion of gases.svg|thumb|The cross section of an alveolus with capillaries is shown. Part of the cross section is magnified to show diffusion of oxygen gas and carbon dioxide through type I cells and capillary cells.]]
[[File:Gas exchange in the aveolus.svg|thumb|Gas exchange in the alveolus]]
Type I cells are the larger of the two cell types; they are thin, flat epithelial lining cells (membranous pneumocytes), that form the structure of the alveoli.<ref name="Knudsen">{{cite journal |last1=Knudsen |first1=L |last2=Ochs |first2=M |title=The micromechanics of lung alveoli: structure and function of surfactant and tissue components. |journal=Histochemistry and Cell Biology |date=December 2018 |volume=150 |issue=6 |pages=661–676 |doi=10.1007/s00418-018-1747-9 |pmid=30390118|pmc=6267411}}</ref> They are squamous (giving more surface area to each cell) and have long cytoplasmic extensions that cover more than 95% of the alveolar surface.<ref name="Saladin"/><ref name="Weinberger">{{cite book |last1=Weinberger |first1=Steven |last2=Cockrill |first2=Barbara |last3=Mandell |first3=Jess | name-list-style = vanc |title=Principles of pulmonary medicine |date=2019 |publisher=Elsevier |isbn=978-0-323-52371-4 |pages=126–129 |edition=Seventh}}</ref>

Type I cells are involved in the process of [[gas exchange]] between the alveoli and [[blood]]. These cells are extremely thin – sometimes only 25&nbsp;nm – the [[electron microscope]] was needed to prove that all alveoli are lined with [[epithelium]]. This thin lining enables a fast [[Diffusion#Diffusion vs. bulk flow|diffusion]] of [[gas exchange]] between the air in the alveoli and the [[blood]] in the surrounding capillaries.

The nucleus of a type I cell occupies a large area of free cytoplasm and its [[organelle]]s are clustered around it reducing the thickness of the cell. This also keeps the thickness of the [[blood-air barrier]] reduced to a minimum.

The cytoplasm in the thin portion contains [[Pinocytosis|pinocytotic vesicles]] which may play a role in the removal of small particulate contaminants from the outer surface. In addition to [[desmosome]]s, all type I alveolar cells have occluding junctions that prevent the leakage of tissue fluid into the alveolar air space.

The relatively low solubility (and hence rate of diffusion) of oxygen necessitates the large internal surface area (about 80 square m [96 square yards]) and very thin walls of the alveoli. Weaving between the capillaries and helping to support them is an [[extracellular matrix]], a meshlike fabric of elastic and collagenous fibres. The collagen fibres, being more rigid, give the wall firmness, while the elastic fibres permit expansion and contraction of the walls during breathing.

Type I pneumocytes are unable to [[mitosis|replicate]] and are susceptible to toxic [[Insult (medical)|insults]]. In the event of damage, type II cells can proliferate and differentiate into type I cells to compensate.<ref name=":0" />

===Type II cells===
Type II cells are cuboidal and much smaller than type I cells.<ref name="Knudsen"/> They are the most numerous cells in the alveoli, yet do not cover as much surface area as the squamous type I cells.<ref name=":0">{{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=1035 |editor-last2=Standring |editor-first2=Susan |editor-last3=Anhand |editor-first3=Neel}}</ref> Type II cells (granulous pneumocytes) in the alveolar wall contain secretory [[organelle]]s known as [[lamellar bodies]] or lamellar granules, that fuse with the cell membranes and secrete [[pulmonary surfactant]]. This surfactant is a film of fatty substances, a group of [[phospholipid]]s that reduce alveolar [[surface tension]]. The phospholipids are stored in the lamellar bodies. Without this coating, the alveoli would collapse. The surfactant is continuously released by [[exocytosis]]. Reinflation of the alveoli following exhalation is made easier by the surfactant, which reduces surface tension in the thin [[Epithelial lining fluid|fluid lining of the alveoli]]. The fluid coating is produced by the body in order to facilitate the transfer of gases between blood and alveolar air, and the type II cells are typically found at the [[blood–air barrier]].<ref>{{cite book | title = Histology, A Text and Atlas | edition = Sixth | date = 2011 | last1 = Ross | first1 = Michael H | last2 = Pawlina | first2 = Wojciech | name-list-style = vanc}}</ref><ref name="pmid11686863">{{cite journal | vauthors = Fehrenbach H | title = Alveolar epithelial type II cell: defender of the alveolus revisited | journal = Respiratory Research | volume = 2 | issue = 1 | pages = 33–46 | date = 2001 | pmid = 11686863 | pmc = 59567 | doi = 10.1186/rr36 | doi-access = free}}</ref>

Type II cells start to develop at about 26 weeks of [[gestation]], secreting small amounts of surfactant. However, adequate amounts of surfactant are not secreted until about 35 weeks of gestation – this is the main reason for increased rates of [[infant respiratory distress syndrome]], which drastically reduces at ages above 35 weeks gestation.

Type II cells are also capable of cellular division, giving rise to more type I and II alveolar cells when the lung tissue is damaged.<ref>{{cite web|url=https://ntp.niehs.nih.gov/nnl/respiratory/lung/regen/index.htm|title=Lung – Regeneration – Nonneoplastic Lesion Atlas| work = National Toxicology Program | publisher =  National Institute of Environmental Health Sciences, National Institutes of Health, U.S. Department of Health and Human Services |access-date=2018-05-18}}</ref>

''[[MUC1]]'', a human [[gene]] associated with type II pneumocytes, has been identified as a marker in [[lung cancer]].<ref name="pmid9850098">{{cite journal | vauthors = Jarrard JA, Linnoila RI, Lee H, Steinberg SM, Witschi H, Szabo E | title = MUC1 is a novel marker for the type II pneumocyte lineage during lung carcinogenesis | journal = Cancer Research | volume = 58 | issue = 23 | pages = 5582–9 | date = December 1998 | pmid = 9850098 | url = http://cancerres.aacrjournals.org/cgi/pmidlookup?view=long&pmid=9850098}}</ref>

The importance of the type 2 lung alveolar cells in the development of severe respiratory symptoms of COVID-19 and potential mechanisms on how these cells are protected by the SSRIs fluvoxamine and fluoxetine was summarized in a review in April 2022.<ref>{{Cite journal |last1=Mahdi |first1=Mohamed |last2=Hermán |first2=Levente |last3=Réthelyi |first3=János M. |last4=Bálint |first4=Bálint László |date=January 2022 |title=Potential Role of the Antidepressants Fluoxetine and Fluvoxamine in the Treatment of COVID-19 |journal=International Journal of Molecular Sciences |language=en |volume=23 |issue=7 |page=3812 |doi=10.3390/ijms23073812 | pmid=35409171 |pmc=8998734 |issn=1422-0067|doi-access=free}}</ref>

===Alveolar macrophages===
The [[alveolar macrophage]]s reside on the internal luminal surfaces of the alveoli, the alveolar ducts, and the bronchioles. They are mobile scavengers that serve to engulf foreign particles in the lungs, such as dust, bacteria, carbon particles, and blood cells from injuries.<ref>{{cite encyclopedia | chapter = The trachea and the stem bronchi | chapter-url = http://www.britannica.com/EBchecked/topic/483141/pulmonary-alveolus | title = Encyclopædia Britannica | publisher = Encyclopædia Britannica, Inc.}}</ref> They are also called ''pulmonary macrophages'', and ''dust cells''. Alveolar macrophages also play a crucial role in immune responses against viral pathogens in the lungs.<ref name=":1">{{Cite journal |last1=Malainou |first1=Christina |last2=Abdin |first2=Shifaa M. |last3=Lachmann |first3=Nico |last4=Matt |first4=Ulrich |last5=Herold |first5=Susanne |date=2023-10-02 |title=Alveolar macrophages in tissue homeostasis, inflammation, and infection: evolving concepts of therapeutic targeting |url=https://www.jci.org/articles/view/170501 |journal=Journal of Clinical Investigation |language=en |volume=133 |issue=19 |doi=10.1172/JCI170501 |issn=1558-8238 |pmc=10541196 |pmid=37781922}}</ref> They secrete cytokines and chemokines, which recruit and activate other immune cells, initiate type I interferon signaling, and inhibit the nuclear export of viral genomes.<ref name=":1" />