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cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle Function subsection</span> </button> <ul id="toc-Function-sublist" class="vector-toc-list"> <li id="toc-Type_I_cells" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Type_I_cells"> <div class="vector-toc-text"> <span class="vector-toc-numb">3.1</span> <span>Type I cells</span> </div> </a> <ul id="toc-Type_I_cells-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Type_II_cells" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Type_II_cells"> <div class="vector-toc-text"> <span class="vector-toc-numb">3.2</span> <span>Type II cells</span> </div> </a> <ul id="toc-Type_II_cells-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Alveolar_macrophages" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Alveolar_macrophages"> <div class="vector-toc-text"> <span class="vector-toc-numb">3.3</span> <span>Alveolar macrophages</span> </div> </a> <ul id="toc-Alveolar_macrophages-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Clinical_significance" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Clinical_significance"> <div class="vector-toc-text"> <span class="vector-toc-numb">4</span> <span>Clinical significance</span> </div> </a> <button aria-controls="toc-Clinical_significance-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle Clinical significance subsection</span> </button> <ul id="toc-Clinical_significance-sublist" class="vector-toc-list"> <li id="toc-Diseases" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Diseases"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.1</span> <span>Diseases</span> </div> </a> <ul id="toc-Diseases-sublist" class="vector-toc-list"> <li id="toc-Surfactant" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Surfactant"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.1.1</span> <span>Surfactant</span> </div> </a> <ul id="toc-Surfactant-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Inflammation" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Inflammation"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.1.2</span> <span>Inflammation</span> </div> </a> <ul id="toc-Inflammation-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Structural" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Structural"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.1.3</span> <span>Structural</span> </div> </a> <ul id="toc-Structural-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Fluid" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Fluid"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.1.4</span> <span>Fluid</span> </div> </a> <ul id="toc-Fluid-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Coronavirus" class="vector-toc-list-item vector-toc-level-3"> <a class="vector-toc-link" href="#Coronavirus"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.1.5</span> <span>Coronavirus</span> </div> </a> <ul id="toc-Coronavirus-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> </ul> </li> <li id="toc-Additional_images" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Additional_images"> <div class="vector-toc-text"> <span class="vector-toc-numb">5</span> <span>Additional images</span> </div> </a> <ul id="toc-Additional_images-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-See_also" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#See_also"> <div class="vector-toc-text"> <span class="vector-toc-numb">6</span> <span>See also</span> </div> </a> <ul id="toc-See_also-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-References" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#References"> <div class="vector-toc-text"> <span class="vector-toc-numb">7</span> <span>References</span> </div> </a> <ul id="toc-References-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-External_links" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#External_links"> <div class="vector-toc-text"> <span class="vector-toc-numb">8</span> <span>External links</span> </div> </a> <ul id="toc-External_links-sublist" class="vector-toc-list"> </ul> </li> </ul> </div> </div> </nav> </div> </div> <div class="mw-content-container"> <main id="content" class="mw-body"> <header class="mw-body-header vector-page-titlebar"> <nav aria-label="Contents" class="vector-toc-landmark"> <div id="vector-page-titlebar-toc" class="vector-dropdown vector-page-titlebar-toc vector-button-flush-left" title="Table of Contents" > <input type="checkbox" id="vector-page-titlebar-toc-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-vector-page-titlebar-toc" class="vector-dropdown-checkbox " aria-label="Toggle the table of contents" > <label id="vector-page-titlebar-toc-label" for="vector-page-titlebar-toc-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--icon-only " aria-hidden="true" ><span class="vector-icon mw-ui-icon-listBullet mw-ui-icon-wikimedia-listBullet"></span> <span class="vector-dropdown-label-text">Toggle the table of contents</span> </label> <div class="vector-dropdown-content"> <div id="vector-page-titlebar-toc-unpinned-container" class="vector-unpinned-container"> </div> </div> </div> </nav> <h1 id="firstHeading" class="firstHeading mw-first-heading"><span class="mw-page-title-main">Pulmonary alveolus</span></h1> <div id="p-lang-btn" class="vector-dropdown mw-portlet mw-portlet-lang" > <input type="checkbox" id="p-lang-btn-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-p-lang-btn" class="vector-dropdown-checkbox mw-interlanguage-selector" aria-label="Go to an article in another language. Available in 60 languages" > <label id="p-lang-btn-label" for="p-lang-btn-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--action-progressive mw-portlet-lang-heading-60" aria-hidden="true" ><span class="vector-icon mw-ui-icon-language-progressive mw-ui-icon-wikimedia-language-progressive"></span> <span class="vector-dropdown-label-text">60 languages</span> </label> <div class="vector-dropdown-content"> <div class="vector-menu-content"> <ul class="vector-menu-content-list"> <li class="interlanguage-link interwiki-als mw-list-item"><a href="https://als.wikipedia.org/wiki/Lungenbl%C3%A4schen" title="Lungenbläschen – Alemannic" lang="gsw" hreflang="gsw" data-title="Lungenbläschen" data-language-autonym="Alemannisch" data-language-local-name="Alemannic" class="interlanguage-link-target"><span>Alemannisch</span></a></li><li class="interlanguage-link interwiki-ar mw-list-item"><a href="https://ar.wikipedia.org/wiki/%D8%AD%D9%88%D9%8A%D8%B5%D9%84%D8%A9_%D9%87%D9%88%D8%A7%D8%A6%D9%8A%D8%A9" title="حويصلة هوائية – Arabic" lang="ar" hreflang="ar" data-title="حويصلة هوائية" data-language-autonym="العربية" data-language-local-name="Arabic" class="interlanguage-link-target"><span>العربية</span></a></li><li class="interlanguage-link interwiki-an mw-list-item"><a href="https://an.wikipedia.org/wiki/Alveolo_pulmonar" title="Alveolo pulmonar – Aragonese" lang="an" hreflang="an" data-title="Alveolo pulmonar" data-language-autonym="Aragonés" data-language-local-name="Aragonese" class="interlanguage-link-target"><span>Aragonés</span></a></li><li class="interlanguage-link interwiki-az mw-list-item"><a href="https://az.wikipedia.org/wiki/Alveol" title="Alveol – Azerbaijani" lang="az" hreflang="az" data-title="Alveol" data-language-autonym="Azərbaycanca" data-language-local-name="Azerbaijani" class="interlanguage-link-target"><span>Azərbaycanca</span></a></li><li class="interlanguage-link interwiki-bn mw-list-item"><a href="https://bn.wikipedia.org/wiki/%E0%A6%AB%E0%A7%81%E0%A6%B8%E0%A6%AB%E0%A7%81%E0%A6%B8%E0%A7%80%E0%A6%AF%E0%A6%BC_%E0%A6%AC%E0%A6%BE%E0%A6%AF%E0%A6%BC%E0%A7%81%E0%A6%A5%E0%A6%B2%E0%A6%BF" title="ফুসফুসীয় বায়ুথলি – Bangla" lang="bn" hreflang="bn" data-title="ফুসফুসীয় বায়ুথলি" data-language-autonym="বাংলা" data-language-local-name="Bangla" class="interlanguage-link-target"><span>বাংলা</span></a></li><li class="interlanguage-link interwiki-be mw-list-item"><a href="https://be.wikipedia.org/wiki/%D0%9B%D1%91%D0%B3%D0%B0%D1%87%D0%BD%D0%B0%D1%8F_%D0%B0%D0%BB%D1%8C%D0%B2%D0%B5%D0%BE%D0%BB%D0%B0" title="Лёгачная альвеола – Belarusian" lang="be" hreflang="be" data-title="Лёгачная альвеола" data-language-autonym="Беларуская" data-language-local-name="Belarusian" class="interlanguage-link-target"><span>Беларуская</span></a></li><li class="interlanguage-link interwiki-bg mw-list-item"><a href="https://bg.wikipedia.org/wiki/%D0%90%D0%BB%D0%B2%D0%B5%D0%BE%D0%BB%D0%B0" title="Алвеола – Bulgarian" lang="bg" hreflang="bg" data-title="Алвеола" data-language-autonym="Български" data-language-local-name="Bulgarian" class="interlanguage-link-target"><span>Български</span></a></li><li class="interlanguage-link interwiki-bo mw-list-item"><a href="https://bo.wikipedia.org/wiki/%E0%BD%A2%E0%BE%B3%E0%BD%B4%E0%BD%84%E0%BC%8B%E0%BD%A2%E0%BE%90%E0%BE%B1%E0%BD%A3%E0%BC%8D" title="རླུང་རྐྱལ། – Tibetan" lang="bo" hreflang="bo" data-title="རླུང་རྐྱལ།" data-language-autonym="བོད་ཡིག" data-language-local-name="Tibetan" class="interlanguage-link-target"><span>བོད་ཡིག</span></a></li><li class="interlanguage-link interwiki-bs mw-list-item"><a href="https://bs.wikipedia.org/wiki/Plu%C4%87na_alveola" title="Plućna alveola – Bosnian" lang="bs" hreflang="bs" data-title="Plućna alveola" data-language-autonym="Bosanski" data-language-local-name="Bosnian" class="interlanguage-link-target"><span>Bosanski</span></a></li><li class="interlanguage-link interwiki-ca mw-list-item"><a href="https://ca.wikipedia.org/wiki/Alv%C3%A8ol" title="Alvèol – Catalan" lang="ca" hreflang="ca" data-title="Alvèol" data-language-autonym="Català" data-language-local-name="Catalan" class="interlanguage-link-target"><span>Català</span></a></li><li class="interlanguage-link interwiki-cs mw-list-item"><a href="https://cs.wikipedia.org/wiki/Plicn%C3%AD_skl%C3%ADpek" title="Plicní sklípek – Czech" lang="cs" hreflang="cs" data-title="Plicní sklípek" data-language-autonym="Čeština" data-language-local-name="Czech" class="interlanguage-link-target"><span>Čeština</span></a></li><li class="interlanguage-link interwiki-cy mw-list-item"><a href="https://cy.wikipedia.org/wiki/Alfeoli" title="Alfeoli – Welsh" lang="cy" hreflang="cy" data-title="Alfeoli" data-language-autonym="Cymraeg" data-language-local-name="Welsh" class="interlanguage-link-target"><span>Cymraeg</span></a></li><li class="interlanguage-link interwiki-da mw-list-item"><a href="https://da.wikipedia.org/wiki/Alveol" title="Alveol – Danish" lang="da" hreflang="da" data-title="Alveol" data-language-autonym="Dansk" data-language-local-name="Danish" class="interlanguage-link-target"><span>Dansk</span></a></li><li class="interlanguage-link interwiki-de mw-list-item"><a href="https://de.wikipedia.org/wiki/Lungenbl%C3%A4schen" title="Lungenbläschen – German" lang="de" hreflang="de" data-title="Lungenbläschen" data-language-autonym="Deutsch" data-language-local-name="German" class="interlanguage-link-target"><span>Deutsch</span></a></li><li class="interlanguage-link interwiki-dv mw-list-item"><a href="https://dv.wikipedia.org/wiki/%DE%86%DE%AA%DE%8B%DE%A8_%DE%88%DE%A6%DE%87%DE%A8%DE%86%DE%AE%DE%8C%DE%A6%DE%85%DE%AA" title="ކުދި ވައިކޮތަޅު – Divehi" lang="dv" hreflang="dv" data-title="ކުދި ވައިކޮތަޅު" data-language-autonym="ދިވެހިބަސް" data-language-local-name="Divehi" class="interlanguage-link-target"><span>ދިވެހިބަސް</span></a></li><li class="interlanguage-link interwiki-et mw-list-item"><a href="https://et.wikipedia.org/wiki/Kopsualveool" title="Kopsualveool – Estonian" lang="et" hreflang="et" data-title="Kopsualveool" data-language-autonym="Eesti" data-language-local-name="Estonian" class="interlanguage-link-target"><span>Eesti</span></a></li><li class="interlanguage-link interwiki-el mw-list-item"><a href="https://el.wikipedia.org/wiki/%CE%9A%CF%85%CF%88%CE%B5%CE%BB%CE%AF%CE%B4%CE%B1" title="Κυψελίδα – Greek" lang="el" hreflang="el" data-title="Κυψελίδα" data-language-autonym="Ελληνικά" data-language-local-name="Greek" class="interlanguage-link-target"><span>Ελληνικά</span></a></li><li class="interlanguage-link interwiki-es mw-list-item"><a href="https://es.wikipedia.org/wiki/Alv%C3%A9olo_pulmonar" title="Alvéolo pulmonar – Spanish" lang="es" hreflang="es" data-title="Alvéolo pulmonar" data-language-autonym="Español" data-language-local-name="Spanish" class="interlanguage-link-target"><span>Español</span></a></li><li class="interlanguage-link interwiki-eo mw-list-item"><a href="https://eo.wikipedia.org/wiki/Pulma_alveolo" title="Pulma alveolo – Esperanto" lang="eo" hreflang="eo" data-title="Pulma alveolo" data-language-autonym="Esperanto" data-language-local-name="Esperanto" class="interlanguage-link-target"><span>Esperanto</span></a></li><li class="interlanguage-link interwiki-eu mw-list-item"><a href="https://eu.wikipedia.org/wiki/Albeolo" title="Albeolo – Basque" lang="eu" hreflang="eu" data-title="Albeolo" data-language-autonym="Euskara" data-language-local-name="Basque" class="interlanguage-link-target"><span>Euskara</span></a></li><li class="interlanguage-link interwiki-fa mw-list-item"><a href="https://fa.wikipedia.org/wiki/%DA%A9%DB%8C%D8%B3%D9%87_%D9%87%D9%88%D8%A7%DB%8C%DB%8C" title="کیسه هوایی – Persian" lang="fa" hreflang="fa" data-title="کیسه هوایی" data-language-autonym="فارسی" data-language-local-name="Persian" class="interlanguage-link-target"><span>فارسی</span></a></li><li class="interlanguage-link interwiki-fr mw-list-item"><a href="https://fr.wikipedia.org/wiki/Alv%C3%A9ole_pulmonaire" title="Alvéole pulmonaire – French" lang="fr" hreflang="fr" data-title="Alvéole pulmonaire" data-language-autonym="Français" data-language-local-name="French" class="interlanguage-link-target"><span>Français</span></a></li><li class="interlanguage-link interwiki-gl mw-list-item"><a href="https://gl.wikipedia.org/wiki/Alv%C3%A9olo_pulmonar" title="Alvéolo pulmonar – Galician" lang="gl" hreflang="gl" data-title="Alvéolo pulmonar" data-language-autonym="Galego" data-language-local-name="Galician" class="interlanguage-link-target"><span>Galego</span></a></li><li class="interlanguage-link interwiki-inh mw-list-item"><a href="https://inh.wikipedia.org/wiki/%D0%9F%D0%B5%D1%85%D0%BA%D0%B8%D0%B9_%D0%B0%D0%BB%D1%8C%D0%B2%D0%B5%D0%BE%D0%BB%D0%B0" title="Пехкий альвеола – Ingush" lang="inh" hreflang="inh" data-title="Пехкий альвеола" data-language-autonym="ГӀалгӀай" data-language-local-name="Ingush" class="interlanguage-link-target"><span>ГӀалгӀай</span></a></li><li class="interlanguage-link interwiki-ko mw-list-item"><a href="https://ko.wikipedia.org/wiki/%ED%8F%90%ED%8F%AC" title="폐포 – Korean" lang="ko" hreflang="ko" data-title="폐포" data-language-autonym="한국어" data-language-local-name="Korean" class="interlanguage-link-target"><span>한국어</span></a></li><li class="interlanguage-link interwiki-hy mw-list-item"><a href="https://hy.wikipedia.org/wiki/%D4%B9%D5%B8%D6%84%D5%A1%D5%A2%D5%B7%D5%BF%D5%A5%D6%80" title="Թոքաբշտեր – Armenian" lang="hy" hreflang="hy" data-title="Թոքաբշտեր" data-language-autonym="Հայերեն" data-language-local-name="Armenian" class="interlanguage-link-target"><span>Հայերեն</span></a></li><li class="interlanguage-link interwiki-id mw-list-item"><a href="https://id.wikipedia.org/wiki/Alveolus" title="Alveolus – Indonesian" lang="id" hreflang="id" data-title="Alveolus" data-language-autonym="Bahasa Indonesia" data-language-local-name="Indonesian" class="interlanguage-link-target"><span>Bahasa Indonesia</span></a></li><li class="interlanguage-link interwiki-ik mw-list-item"><a href="https://ik.wikipedia.org/wiki/Si%E1%B8%B7atuuti%C5%8Bi_puvaum" title="Siḷatuutiŋi puvaum – Inupiaq" lang="ik" hreflang="ik" data-title="Siḷatuutiŋi puvaum" data-language-autonym="Iñupiatun" data-language-local-name="Inupiaq" class="interlanguage-link-target"><span>Iñupiatun</span></a></li><li class="interlanguage-link interwiki-it mw-list-item"><a href="https://it.wikipedia.org/wiki/Alveolo_polmonare" title="Alveolo polmonare – Italian" lang="it" hreflang="it" data-title="Alveolo polmonare" data-language-autonym="Italiano" data-language-local-name="Italian" class="interlanguage-link-target"><span>Italiano</span></a></li><li class="interlanguage-link interwiki-he mw-list-item"><a href="https://he.wikipedia.org/wiki/%D7%A0%D7%90%D7%93%D7%99%D7%AA" title="נאדית – Hebrew" lang="he" hreflang="he" data-title="נאדית" data-language-autonym="עברית" data-language-local-name="Hebrew" class="interlanguage-link-target"><span>עברית</span></a></li><li class="interlanguage-link interwiki-kk mw-list-item"><a href="https://kk.wikipedia.org/wiki/%D3%A8%D0%BA%D0%BF%D0%B5_%D0%B0%D0%BB%D1%8C%D0%B2%D0%B5%D0%BE%D0%BB%D0%B0%D1%81%D1%8B" title="Өкпе альвеоласы – Kazakh" lang="kk" hreflang="kk" data-title="Өкпе альвеоласы" data-language-autonym="Қазақша" data-language-local-name="Kazakh" class="interlanguage-link-target"><span>Қазақша</span></a></li><li class="interlanguage-link interwiki-ht mw-list-item"><a href="https://ht.wikipedia.org/wiki/Alvey%C3%B2l" title="Alveyòl – Haitian Creole" lang="ht" hreflang="ht" data-title="Alveyòl" data-language-autonym="Kreyòl ayisyen" data-language-local-name="Haitian Creole" class="interlanguage-link-target"><span>Kreyòl ayisyen</span></a></li><li class="interlanguage-link interwiki-ku mw-list-item"><a href="https://ku.wikipedia.org/wiki/Peqoka_pi%C5%9Fik%C3%AA" title="Peqoka pişikê – Kurdish" lang="ku" hreflang="ku" data-title="Peqoka pişikê" data-language-autonym="Kurdî" data-language-local-name="Kurdish" class="interlanguage-link-target"><span>Kurdî</span></a></li><li class="interlanguage-link interwiki-ky mw-list-item"><a href="https://ky.wikipedia.org/wiki/%D0%90%D0%BB%D1%8C%D0%B2%D0%B5%D0%BE%D0%BB%D0%B0%D0%BB%D1%8B%D0%BA_%D3%A9%D0%BF%D0%BA%D3%A9%D0%BB%D3%A9%D1%80" title="Альвеолалык өпкөлөр – Kyrgyz" lang="ky" hreflang="ky" data-title="Альвеолалык өпкөлөр" data-language-autonym="Кыргызча" data-language-local-name="Kyrgyz" class="interlanguage-link-target"><span>Кыргызча</span></a></li><li class="interlanguage-link interwiki-la mw-list-item"><a href="https://la.wikipedia.org/wiki/Alveolus_pulmonis" title="Alveolus pulmonis – Latin" lang="la" hreflang="la" data-title="Alveolus pulmonis" data-language-autonym="Latina" data-language-local-name="Latin" class="interlanguage-link-target"><span>Latina</span></a></li><li class="interlanguage-link interwiki-lt mw-list-item"><a href="https://lt.wikipedia.org/wiki/Plau%C4%8Di%C5%B3_alveol%C4%97" title="Plaučių alveolė – Lithuanian" lang="lt" hreflang="lt" data-title="Plaučių alveolė" data-language-autonym="Lietuvių" data-language-local-name="Lithuanian" class="interlanguage-link-target"><span>Lietuvių</span></a></li><li class="interlanguage-link interwiki-mk mw-list-item"><a href="https://mk.wikipedia.org/wiki/%D0%90%D0%BB%D0%B2%D0%B5%D0%BE%D0%BB%D0%B0" title="Алвеола – Macedonian" lang="mk" hreflang="mk" data-title="Алвеола" data-language-autonym="Македонски" data-language-local-name="Macedonian" class="interlanguage-link-target"><span>Македонски</span></a></li><li class="interlanguage-link interwiki-mdf mw-list-item"><a href="https://mdf.wikipedia.org/wiki/%D0%90%D0%BB%D1%8C%D0%B2%D0%B5%D0%BE%D0%BB%D0%B0%D1%82%D0%BD%D0%B5" title="Альвеолатне – Moksha" lang="mdf" hreflang="mdf" data-title="Альвеолатне" data-language-autonym="Мокшень" data-language-local-name="Moksha" class="interlanguage-link-target"><span>Мокшень</span></a></li><li class="interlanguage-link interwiki-nl mw-list-item"><a href="https://nl.wikipedia.org/wiki/Longblaasje" title="Longblaasje – Dutch" lang="nl" hreflang="nl" data-title="Longblaasje" data-language-autonym="Nederlands" data-language-local-name="Dutch" class="interlanguage-link-target"><span>Nederlands</span></a></li><li class="interlanguage-link interwiki-ja mw-list-item"><a href="https://ja.wikipedia.org/wiki/%E8%82%BA%E8%83%9E" title="肺胞 – Japanese" lang="ja" hreflang="ja" data-title="肺胞" data-language-autonym="日本語" data-language-local-name="Japanese" class="interlanguage-link-target"><span>日本語</span></a></li><li class="interlanguage-link interwiki-no mw-list-item"><a href="https://no.wikipedia.org/wiki/Alveol" title="Alveol – Norwegian Bokmål" lang="nb" hreflang="nb" data-title="Alveol" data-language-autonym="Norsk bokmål" data-language-local-name="Norwegian Bokmål" class="interlanguage-link-target"><span>Norsk bokmål</span></a></li><li class="interlanguage-link interwiki-pl mw-list-item"><a href="https://pl.wikipedia.org/wiki/P%C4%99cherzyki_p%C5%82ucne" title="Pęcherzyki płucne – Polish" lang="pl" hreflang="pl" data-title="Pęcherzyki płucne" data-language-autonym="Polski" data-language-local-name="Polish" class="interlanguage-link-target"><span>Polski</span></a></li><li class="interlanguage-link interwiki-pt mw-list-item"><a href="https://pt.wikipedia.org/wiki/Alv%C3%A9olo_pulmonar" title="Alvéolo pulmonar – Portuguese" lang="pt" hreflang="pt" data-title="Alvéolo pulmonar" data-language-autonym="Português" data-language-local-name="Portuguese" class="interlanguage-link-target"><span>Português</span></a></li><li class="interlanguage-link interwiki-ro mw-list-item"><a href="https://ro.wikipedia.org/wiki/Alveol%C4%83_pulmonar%C4%83" title="Alveolă pulmonară – Romanian" lang="ro" hreflang="ro" data-title="Alveolă pulmonară" data-language-autonym="Română" data-language-local-name="Romanian" class="interlanguage-link-target"><span>Română</span></a></li><li class="interlanguage-link interwiki-qu mw-list-item"><a href="https://qu.wikipedia.org/wiki/Samana_p%27urucha" title="Samana p'urucha – Quechua" lang="qu" hreflang="qu" data-title="Samana p'urucha" data-language-autonym="Runa Simi" data-language-local-name="Quechua" class="interlanguage-link-target"><span>Runa Simi</span></a></li><li class="interlanguage-link interwiki-ru mw-list-item"><a href="https://ru.wikipedia.org/wiki/%D0%9B%D1%91%D0%B3%D0%BE%D1%87%D0%BD%D0%B0%D1%8F_%D0%B0%D0%BB%D1%8C%D0%B2%D0%B5%D0%BE%D0%BB%D0%B0" title="Лёгочная альвеола – Russian" lang="ru" hreflang="ru" data-title="Лёгочная альвеола" data-language-autonym="Русский" data-language-local-name="Russian" class="interlanguage-link-target"><span>Русский</span></a></li><li class="interlanguage-link interwiki-szy mw-list-item"><a href="https://szy.wikipedia.org/wiki/bala%27-besi" title="bala'-besi – Sakizaya" lang="szy" hreflang="szy" data-title="bala'-besi" data-language-autonym="Sakizaya" data-language-local-name="Sakizaya" class="interlanguage-link-target"><span>Sakizaya</span></a></li><li class="interlanguage-link interwiki-simple mw-list-item"><a href="https://simple.wikipedia.org/wiki/Alveolus" title="Alveolus – Simple English" lang="en-simple" hreflang="en-simple" data-title="Alveolus" data-language-autonym="Simple English" data-language-local-name="Simple English" class="interlanguage-link-target"><span>Simple English</span></a></li><li class="interlanguage-link interwiki-sk mw-list-item"><a href="https://sk.wikipedia.org/wiki/P%C4%BE%C3%BAcny_mech%C3%BArik" title="Pľúcny mechúrik – Slovak" lang="sk" hreflang="sk" data-title="Pľúcny mechúrik" data-language-autonym="Slovenčina" data-language-local-name="Slovak" class="interlanguage-link-target"><span>Slovenčina</span></a></li><li class="interlanguage-link interwiki-sl mw-list-item"><a href="https://sl.wikipedia.org/wiki/Plju%C4%8Dni_me%C5%A1i%C4%8Dek" title="Pljučni mešiček – Slovenian" lang="sl" hreflang="sl" data-title="Pljučni mešiček" data-language-autonym="Slovenščina" data-language-local-name="Slovenian" class="interlanguage-link-target"><span>Slovenščina</span></a></li><li class="interlanguage-link interwiki-ckb mw-list-item"><a href="https://ckb.wikipedia.org/wiki/%D8%B3%DB%8C%DA%A9%DA%B5%D8%AF%D8%A7%D9%86%DB%86%DA%86%DA%A9%DB%95%DB%8C_%DA%BE%DB%95%D9%88%D8%A7" title="سیکڵدانۆچکەی ھەوا – Central Kurdish" lang="ckb" hreflang="ckb" data-title="سیکڵدانۆچکەی ھەوا" data-language-autonym="کوردی" data-language-local-name="Central Kurdish" class="interlanguage-link-target"><span>کوردی</span></a></li><li class="interlanguage-link interwiki-sr mw-list-item"><a href="https://sr.wikipedia.org/wiki/%D0%9F%D0%BB%D1%83%D1%9B%D0%BD%D0%B0_%D0%B0%D0%BB%D0%B2%D0%B5%D0%BE%D0%BB%D0%B0" title="Плућна алвеола – Serbian" lang="sr" hreflang="sr" data-title="Плућна алвеола" data-language-autonym="Српски / srpski" data-language-local-name="Serbian" class="interlanguage-link-target"><span>Српски / srpski</span></a></li><li class="interlanguage-link interwiki-sh mw-list-item"><a href="https://sh.wikipedia.org/wiki/Plu%C4%87na_alveola" title="Plućna alveola – Serbo-Croatian" lang="sh" hreflang="sh" data-title="Plućna alveola" data-language-autonym="Srpskohrvatski / српскохрватски" data-language-local-name="Serbo-Croatian" class="interlanguage-link-target"><span>Srpskohrvatski / српскохрватски</span></a></li><li class="interlanguage-link interwiki-fi mw-list-item"><a href="https://fi.wikipedia.org/wiki/Keuhkorakkula" title="Keuhkorakkula – Finnish" lang="fi" hreflang="fi" data-title="Keuhkorakkula" data-language-autonym="Suomi" data-language-local-name="Finnish" class="interlanguage-link-target"><span>Suomi</span></a></li><li class="interlanguage-link interwiki-sv mw-list-item"><a href="https://sv.wikipedia.org/wiki/Alveol_(luftr%C3%B6rsf%C3%B6rgrening)" title="Alveol (luftrörsförgrening) – Swedish" lang="sv" hreflang="sv" data-title="Alveol (luftrörsförgrening)" data-language-autonym="Svenska" data-language-local-name="Swedish" class="interlanguage-link-target"><span>Svenska</span></a></li><li class="interlanguage-link interwiki-tr mw-list-item"><a href="https://tr.wikipedia.org/wiki/Alveol" title="Alveol – Turkish" lang="tr" hreflang="tr" data-title="Alveol" data-language-autonym="Türkçe" data-language-local-name="Turkish" class="interlanguage-link-target"><span>Türkçe</span></a></li><li class="interlanguage-link interwiki-uk mw-list-item"><a href="https://uk.wikipedia.org/wiki/%D0%9B%D0%B5%D0%B3%D0%B5%D0%BD%D0%B5%D0%B2%D0%B0_%D0%B0%D0%BB%D1%8C%D0%B2%D0%B5%D0%BE%D0%BB%D0%B0" title="Легенева альвеола – Ukrainian" lang="uk" hreflang="uk" data-title="Легенева альвеола" data-language-autonym="Українська" data-language-local-name="Ukrainian" class="interlanguage-link-target"><span>Українська</span></a></li><li class="interlanguage-link interwiki-wuu mw-list-item"><a href="https://wuu.wikipedia.org/wiki/%E8%82%BA%E6%B3%A1" title="肺泡 – Wu" lang="wuu" hreflang="wuu" data-title="肺泡" data-language-autonym="吴语" data-language-local-name="Wu" class="interlanguage-link-target"><span>吴语</span></a></li><li class="interlanguage-link interwiki-zh-yue mw-list-item"><a href="https://zh-yue.wikipedia.org/wiki/%E8%82%BA%E6%B3%A1" title="肺泡 – Cantonese" lang="yue" hreflang="yue" data-title="肺泡" data-language-autonym="粵語" data-language-local-name="Cantonese" class="interlanguage-link-target"><span>粵語</span></a></li><li class="interlanguage-link interwiki-zh mw-list-item"><a href="https://zh.wikipedia.org/wiki/%E8%82%BA%E6%B3%A1" title="肺泡 – Chinese" lang="zh" hreflang="zh" data-title="肺泡" data-language-autonym="中文" data-language-local-name="Chinese" class="interlanguage-link-target"><span>中文</span></a></li> </ul> <div class="after-portlet 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dir="ltr"><div class="shortdescription nomobile noexcerpt noprint searchaux" style="display:none">Hollow cavity found in the lungs</div> <p class="mw-empty-elt"> </p> <style data-mw-deduplicate="TemplateStyles:r1236090951">.mw-parser-output .hatnote{font-style:italic}.mw-parser-output div.hatnote{padding-left:1.6em;margin-bottom:0.5em}.mw-parser-output .hatnote i{font-style:normal}.mw-parser-output .hatnote+link+.hatnote{margin-top:-0.5em}@media print{body.ns-0 .mw-parser-output .hatnote{display:none!important}}</style><div role="note" class="hatnote navigation-not-searchable">For other uses, see <a href="/wiki/Alveolus_(disambiguation)" class="mw-redirect" title="Alveolus (disambiguation)">Alveolus</a>.</div> <style data-mw-deduplicate="TemplateStyles:r1257001546">.mw-parser-output .infobox-subbox{padding:0;border:none;margin:-3px;width:auto;min-width:100%;font-size:100%;clear:none;float:none;background-color:transparent}.mw-parser-output .infobox-3cols-child{margin:auto}.mw-parser-output .infobox .navbar{font-size:100%}@media screen{html.skin-theme-clientpref-night .mw-parser-output .infobox-full-data:not(.notheme)>div:not(.notheme)[style]{background:#1f1f23!important;color:#f8f9fa}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .infobox-full-data:not(.notheme) div:not(.notheme){background:#1f1f23!important;color:#f8f9fa}}@media(min-width:640px){body.skin--responsive .mw-parser-output .infobox-table{display:table!important}body.skin--responsive .mw-parser-output .infobox-table>caption{display:table-caption!important}body.skin--responsive .mw-parser-output .infobox-table>tbody{display:table-row-group}body.skin--responsive .mw-parser-output .infobox-table tr{display:table-row!important}body.skin--responsive .mw-parser-output .infobox-table th,body.skin--responsive .mw-parser-output .infobox-table td{padding-left:inherit;padding-right:inherit}}</style><table class="infobox"><tbody><tr><th colspan="2" class="infobox-above" style="background-color:dimgray; color: white">Pulmonary alveolus</th></tr><tr><td colspan="2" class="infobox-image"><span class="mw-default-size" typeof="mw:File/Frameless"><a href="/wiki/File:Alveolus_diagram.svg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/4/46/Alveolus_diagram.svg/250px-Alveolus_diagram.svg.png" decoding="async" width="250" height="189" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/4/46/Alveolus_diagram.svg/375px-Alveolus_diagram.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/4/46/Alveolus_diagram.svg/500px-Alveolus_diagram.svg.png 2x" data-file-width="465" data-file-height="352" /></a></span><div class="infobox-caption">The alveoli</div></td></tr><tr><th colspan="2" class="infobox-header" style="background-color: #efefef">Details</th></tr><tr><th scope="row" class="infobox-label" style="padding-right:0.25em"><a href="/wiki/Organ_system" title="Organ system">System</a></th><td class="infobox-data"><a href="/wiki/Respiratory_system" title="Respiratory system">Respiratory system</a></td></tr><tr><th scope="row" class="infobox-label" style="padding-right:0.25em">Location</th><td class="infobox-data"><a href="/wiki/Lung" title="Lung">Lung</a></td></tr><tr><th colspan="2" class="infobox-header" style="background-color: #efefef">Identifiers</th></tr><tr><th scope="row" class="infobox-label" style="padding-right:0.25em"><a href="/wiki/Latin" title="Latin">Latin</a></th><td class="infobox-data"><i>alveolus pulmonis</i></td></tr><tr><th scope="row" class="infobox-label" style="padding-right:0.25em"><a href="/wiki/Medical_Subject_Headings" title="Medical Subject Headings">MeSH</a></th><td class="infobox-data"><a rel="nofollow" class="external text" href="https://meshb.nlm.nih.gov/record/ui?ui=D011650">D011650</a></td></tr><tr><th scope="row" class="infobox-label" style="padding-right:0.25em"><a href="/wiki/Terminologia_Histologica" title="Terminologia Histologica">TH</a></th><td class="infobox-data"><a rel="nofollow" class="external text" href="https://ifaa.unifr.ch/Public/EntryPage/PDF/TH%20Chapter%20H3.05.pdf">H3.05.02.0.00026 </a></td></tr><tr><th scope="row" class="infobox-label" style="padding-right:0.25em"><a href="/wiki/Foundational_Model_of_Anatomy" title="Foundational Model of Anatomy">FMA</a></th><td class="infobox-data"><a rel="nofollow" class="external text" href="https://bioportal.bioontology.org/ontologies/FMA/?p=classes&conceptid=http%3A%2F%2Fpurl.org%2Fsig%2Font%2Ffma%2Ffma7318">7318</a></td></tr><tr><td colspan="2" class="infobox-below"><a href="/wiki/Anatomical_terminology" title="Anatomical terminology">Anatomical terminology</a><div style="text-align: right;"><small class="noprint">[<a href="https://www.wikidata.org/wiki/Q191341" class="extiw" title="d:Q191341">edit on Wikidata</a>]</small></div></td></tr></tbody></table> <p>A <b>pulmonary alveolus</b> (<abbr title="plural">pl.</abbr><span style="white-space:nowrap;"> <b>alveoli</b></span>; from <a href="/wiki/Latin_language" class="mw-redirect" title="Latin language">Latin</a> <i> <i lang="la"><a href="https://en.wiktionary.org/wiki/alveolus#Latin" class="extiw" title="wikt:alveolus">alveolus</a></i></i> 'little cavity'), also called an <b>air sac</b> or <b>air space</b>, is one of millions of hollow, distensible cup-shaped cavities in the <a href="/wiki/Lung" title="Lung">lungs</a> where pulmonary gas exchange takes place.<sup id="cite_ref-MeSH1_1-0" class="reference"><a href="#cite_note-MeSH1-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> <a href="/wiki/Oxygen" title="Oxygen">Oxygen</a> is <a href="/wiki/Molecular_diffusion#Biology" title="Molecular diffusion">exchanged</a> for <a href="/wiki/Carbon_dioxide" title="Carbon dioxide">carbon dioxide</a> at the <a href="/wiki/Blood%E2%80%93air_barrier" title="Blood–air barrier">blood–air barrier</a> between the alveolar air and the <a href="/wiki/Pulmonary_circulation" title="Pulmonary circulation">pulmonary capillary</a>.<sup id="cite_ref-SEER_2-0" class="reference"><a href="#cite_note-SEER-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> Alveoli make up the functional tissue of the <a href="/wiki/Mammal" title="Mammal">mammalian</a> lungs known as the <a href="/wiki/Lung_parenchyma" class="mw-redirect" title="Lung parenchyma">lung parenchyma</a>, which takes up 90 percent of the total lung volume.<sup id="cite_ref-Knudsen_3-0" class="reference"><a href="#cite_note-Knudsen-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Jones_4-0" class="reference"><a href="#cite_note-Jones-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> </p><p>Alveoli are first located in the <a href="/wiki/Respiratory_bronchiole" class="mw-redirect" title="Respiratory bronchiole">respiratory bronchioles</a> that mark the beginning of the <a href="/wiki/Respiratory_zone" class="mw-redirect" title="Respiratory zone">respiratory zone</a>. They are located sparsely in these bronchioles, line the walls of the <a href="/wiki/Alveolar_duct" class="mw-redirect" title="Alveolar duct">alveolar ducts</a>, and are more numerous in the blind-ended <a href="/wiki/Alveolar_sac" class="mw-redirect" title="Alveolar sac">alveolar sacs</a>.<sup id="cite_ref-Moore_5-0" class="reference"><a href="#cite_note-Moore-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> The <a href="/wiki/Pulmonary_acinus" class="mw-redirect" title="Pulmonary acinus">acini</a> are the basic units of respiration, with <a href="/wiki/Gas_exchange" title="Gas exchange">gas exchange</a> taking place in all the alveoli present.<sup id="cite_ref-6" class="reference"><a href="#cite_note-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> The alveolar membrane is the gas exchange surface, surrounded by a network of <a href="/wiki/Capillary" title="Capillary">capillaries</a>. Oxygen is <a href="/wiki/Diffusion#Diffusion_vs._bulk_flow" title="Diffusion">diffused</a> across the membrane into the capillaries and carbon dioxide is released from the capillaries into the alveoli to be breathed out.<sup id="cite_ref-7" class="reference"><a href="#cite_note-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-8" class="reference"><a href="#cite_note-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup> </p><p>Alveoli are particular to mammalian lungs. Different structures are involved in gas exchange in other vertebrates.<sup id="cite_ref-9" class="reference"><a href="#cite_note-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup> </p> <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="Structure">Structure</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Pulmonary_alveolus&action=edit&section=1" title="Edit section: Structure"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Bronchial_anatomy_with_description.png" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/f/f5/Bronchial_anatomy_with_description.png/310px-Bronchial_anatomy_with_description.png" decoding="async" width="310" height="240" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/f/f5/Bronchial_anatomy_with_description.png/465px-Bronchial_anatomy_with_description.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/f/f5/Bronchial_anatomy_with_description.png/620px-Bronchial_anatomy_with_description.png 2x" data-file-width="1059" data-file-height="821" /></a><figcaption><a href="/wiki/Bronchiole" title="Bronchiole">Bronchial</a> anatomy showing <a href="/wiki/Terminal_bronchioles" class="mw-redirect" title="Terminal bronchioles">terminal bronchioles</a> (BT) leading to <a href="/wiki/Respiratory_bronchiole" class="mw-redirect" title="Respiratory bronchiole">respiratory bronchioles</a> (BR) and alveolar ducts (DA) that open into alveolar sacs containing out pockets of alveoli (A) separated by <a href="/wiki/Alveolar_septum" class="mw-redirect" title="Alveolar septum">alveolar septa</a> (AS)</figcaption></figure> <p>The alveoli are first located in the respiratory bronchioles as scattered outpockets, extending from their lumens. The respiratory bronchioles run for considerable lengths and become increasingly alveolated with side branches of <b>alveolar ducts</b> that become deeply lined with alveoli. The ducts number between two and eleven from each bronchiole.<sup id="cite_ref-Spencer_2_10-0" class="reference"><a href="#cite_note-Spencer_2-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup> Each duct opens into five or six <b>alveolar sacs</b> into which clusters of alveoli open. </p><p>Each terminal respiratory unit is called an <a href="/wiki/Lung#Respiratory_zone" title="Lung">acinus</a> and consists of the respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli. New alveoli continue to form until the age of eight years.<sup id="cite_ref-Moore_5-1" class="reference"><a href="#cite_note-Moore-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> </p><p>A typical pair of <a href="/wiki/Lung" title="Lung">human lungs</a> contains about 480 million alveoli,<sup id="cite_ref-pmid14512270_11-0" class="reference"><a href="#cite_note-pmid14512270-11"><span class="cite-bracket">[</span>11<span class="cite-bracket">]</span></a></sup> providing a total surface area for gas exchange of between 70 and 80 square metres.<sup id="cite_ref-Spencer_2_10-1" class="reference"><a href="#cite_note-Spencer_2-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup> Each alveolus is wrapped in a fine mesh of <a href="/wiki/Capillaries" class="mw-redirect" title="Capillaries">capillaries</a> covering about 70% of its area.<sup id="cite_ref-Saladin_12-0" class="reference"><a href="#cite_note-Saladin-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup> The diameter of an alveolus is between 200 and 500 <a href="/wiki/Micrometre" title="Micrometre">μm</a>.<sup id="cite_ref-Saladin_12-1" class="reference"><a href="#cite_note-Saladin-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Microanatomy">Microanatomy</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Pulmonary_alveolus&action=edit&section=2" title="Edit section: Microanatomy"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>An alveolus consists of an <a href="/wiki/Epithelial" class="mw-redirect" title="Epithelial">epithelial</a> layer of simple <a href="/wiki/Epithelium#Cell_types" title="Epithelium">squamous epithelium</a> (very thin, flattened cells),<sup id="cite_ref-NIH_13-0" class="reference"><a href="#cite_note-NIH-13"><span class="cite-bracket">[</span>13<span class="cite-bracket">]</span></a></sup> and an <a href="/wiki/Extracellular_matrix" title="Extracellular matrix">extracellular matrix</a> surrounded by <a href="/wiki/Capillary" title="Capillary">capillaries</a>. The epithelial lining is part of the alveolar membrane, also known as the respiratory membrane, that allows the <a href="/wiki/Gas_exchange" title="Gas exchange">exchange of gases</a>. The membrane has several layers – a layer of <a href="/wiki/Alveolar_lining_fluid" class="mw-redirect" title="Alveolar lining fluid">alveolar lining fluid</a> that contains <a href="/wiki/Surfactant" title="Surfactant">surfactant</a>, the epithelial layer and its basement membrane; a thin <a href="/wiki/Interstitium" title="Interstitium">interstitial space</a> between the epithelial lining and the capillary membrane; a capillary basement membrane that often fuses with the alveolar basement membrane, and the capillary <a href="/wiki/Endothelial" class="mw-redirect" title="Endothelial">endothelial</a> membrane. The whole membrane however is only between 0.2 <a href="/wiki/Micrometre" title="Micrometre">μm</a> at its thinnest part and 0.6 μm at its thickest.<sup id="cite_ref-Hall_14-0" class="reference"><a href="#cite_note-Hall-14"><span class="cite-bracket">[</span>14<span class="cite-bracket">]</span></a></sup> </p><p>In the <b>alveolar walls</b> there are interconnecting air passages between the alveoli known as the <a href="/wiki/Pores_of_Kohn" title="Pores of Kohn">pores of Kohn</a>. The <b>alveolar septum</b> that separates the alveoli in the alveolar sac contains some <a href="/wiki/Collagen" title="Collagen">collagen fibers</a> and <a href="/wiki/Elastic_fibers" class="mw-redirect" title="Elastic fibers">elastic fibers</a>. The septa also house the enmeshed capillary network that surrounds each alveolus.<sup id="cite_ref-Knudsen_3-1" class="reference"><a href="#cite_note-Knudsen-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> The elastic fibres allow the alveoli to stretch when they fill with air during inhalation. They then spring back during exhalation in order to expel the carbon dioxide-rich air. </p> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Alveolar_sac.JPG" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/6/6c/Alveolar_sac.JPG/220px-Alveolar_sac.JPG" decoding="async" width="220" height="165" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/6/6c/Alveolar_sac.JPG/330px-Alveolar_sac.JPG 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/6/6c/Alveolar_sac.JPG/440px-Alveolar_sac.JPG 2x" data-file-width="2816" data-file-height="2112" /></a><figcaption>A histologic slide of a human alveolar sac</figcaption></figure> <p>There are three major types of <b>alveolar cell</b>. Two types are <b>pneumocytes</b> or <b>pneumonocytes</b> known as type I and type II cells found in the alveolar wall, and a large <a href="/wiki/Phagocyte" title="Phagocyte">phagocytic cell</a> known as an <a href="/wiki/Alveolar_macrophage" title="Alveolar macrophage">alveolar macrophage</a> that moves about in the lumens of the alveoli, and in the connective tissue between them. Type I cells, also called type I pneumocytes, or type I alveolar cells, are squamous, thin and flat and form the structure of the alveoli. Type II cells, also called type II pneumocytes or type II alveolar cells, release <a href="/wiki/Pulmonary_surfactant" title="Pulmonary surfactant">pulmonary surfactant</a> to lower <a href="/wiki/Surface_tension" title="Surface tension">surface tension</a>, and can also <a href="/wiki/Cellular_differentiation" title="Cellular differentiation">differentiate</a> to replace damaged type I cells.<sup id="cite_ref-Saladin_12-2" class="reference"><a href="#cite_note-Saladin-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Naeem_15-0" class="reference"><a href="#cite_note-Naeem-15"><span class="cite-bracket">[</span>15<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Development">Development</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Pulmonary_alveolus&action=edit&section=3" title="Edit section: Development"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Development of the earliest structures that will contain alveoli begins on day 22 and is divided into five stages: embryonic, pseudoglandular, canalicular, saccular, and alveolar stage.<sup id="cite_ref-Embryology,_Pulmonary_16-0" class="reference"><a href="#cite_note-Embryology,_Pulmonary-16"><span class="cite-bracket">[</span>16<span class="cite-bracket">]</span></a></sup> The alveolar stage begins approximately 36 weeks into development. Immature alveoli appear as bulges from the sacculi which invade the primary septa. As the sacculi develop, the protrusions in the primary septa become larger; new septations are longer and thinner and are known as secondary septa.<sup id="cite_ref-Embryology,_Pulmonary_16-1" class="reference"><a href="#cite_note-Embryology,_Pulmonary-16"><span class="cite-bracket">[</span>16<span class="cite-bracket">]</span></a></sup> Secondary septa are responsible for the final division of the sacculi into alveoli. Majority of alveolar division occurs within the first 6 months but continue to develop until 3 years of age. To create a thinner diffusion barrier, the double-layer capillary network fuse into one network, each one closely associated with two alveoli as they develop.<sup id="cite_ref-Embryology,_Pulmonary_16-2" class="reference"><a href="#cite_note-Embryology,_Pulmonary-16"><span class="cite-bracket">[</span>16<span class="cite-bracket">]</span></a></sup> </p><p>In the first three years of life, the enlargement of lungs is a consequence of the increasing number of alveoli; after this point, both the number and size of alveoli increases until the development of lungs finishes at approximately 8 years of age.<sup id="cite_ref-Embryology,_Pulmonary_16-3" class="reference"><a href="#cite_note-Embryology,_Pulmonary-16"><span class="cite-bracket">[</span>16<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Function">Function</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Pulmonary_alveolus&action=edit&section=4" title="Edit section: Function"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:An_annotated_diagram_of_an_alveolus.svg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/a/af/An_annotated_diagram_of_an_alveolus.svg/220px-An_annotated_diagram_of_an_alveolus.svg.png" decoding="async" width="220" height="165" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/a/af/An_annotated_diagram_of_an_alveolus.svg/330px-An_annotated_diagram_of_an_alveolus.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/a/af/An_annotated_diagram_of_an_alveolus.svg/440px-An_annotated_diagram_of_an_alveolus.svg.png 2x" data-file-width="512" data-file-height="384" /></a><figcaption>An annotated diagram of the alveolus</figcaption></figure> <div class="mw-heading mw-heading3"><h3 id="Type_I_cells">Type I cells</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Pulmonary_alveolus&action=edit&section=5" title="Edit section: Type I cells"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Cross_section_of_an_alveolus_and_capillaries_showing_diffusion_of_gases.svg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/1/13/Cross_section_of_an_alveolus_and_capillaries_showing_diffusion_of_gases.svg/220px-Cross_section_of_an_alveolus_and_capillaries_showing_diffusion_of_gases.svg.png" decoding="async" width="220" height="143" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/1/13/Cross_section_of_an_alveolus_and_capillaries_showing_diffusion_of_gases.svg/330px-Cross_section_of_an_alveolus_and_capillaries_showing_diffusion_of_gases.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/1/13/Cross_section_of_an_alveolus_and_capillaries_showing_diffusion_of_gases.svg/440px-Cross_section_of_an_alveolus_and_capillaries_showing_diffusion_of_gases.svg.png 2x" data-file-width="687" data-file-height="447" /></a><figcaption>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.</figcaption></figure> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Gas_exchange_in_the_aveolus.svg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/7/7d/Gas_exchange_in_the_aveolus.svg/220px-Gas_exchange_in_the_aveolus.svg.png" decoding="async" width="220" height="179" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/7/7d/Gas_exchange_in_the_aveolus.svg/330px-Gas_exchange_in_the_aveolus.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/7/7d/Gas_exchange_in_the_aveolus.svg/440px-Gas_exchange_in_the_aveolus.svg.png 2x" data-file-width="390" data-file-height="317" /></a><figcaption>Gas exchange in the alveolus</figcaption></figure> <p>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.<sup id="cite_ref-Knudsen_3-2" class="reference"><a href="#cite_note-Knudsen-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> They are squamous (giving more surface area to each cell) and have long cytoplasmic extensions that cover more than 95% of the alveolar surface.<sup id="cite_ref-Saladin_12-3" class="reference"><a href="#cite_note-Saladin-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-Weinberger_17-0" class="reference"><a href="#cite_note-Weinberger-17"><span class="cite-bracket">[</span>17<span class="cite-bracket">]</span></a></sup> </p><p>Type I cells are involved in the process of <a href="/wiki/Gas_exchange" title="Gas exchange">gas exchange</a> between the alveoli and <a href="/wiki/Blood" title="Blood">blood</a>. These cells are extremely thin – sometimes only 25 nm – the <a href="/wiki/Electron_microscope" title="Electron microscope">electron microscope</a> was needed to prove that all alveoli are lined with <a href="/wiki/Epithelium" title="Epithelium">epithelium</a>. This thin lining enables a fast <a href="/wiki/Diffusion#Diffusion_vs._bulk_flow" title="Diffusion">diffusion</a> of <a href="/wiki/Gas_exchange" title="Gas exchange">gas exchange</a> between the air in the alveoli and the <a href="/wiki/Blood" title="Blood">blood</a> in the surrounding capillaries. </p><p>The nucleus of a type I cell occupies a large area of free cytoplasm and its <a href="/wiki/Organelle" title="Organelle">organelles</a> are clustered around it reducing the thickness of the cell. This also keeps the thickness of the <a href="/wiki/Blood-air_barrier" class="mw-redirect" title="Blood-air barrier">blood-air barrier</a> reduced to a minimum. </p><p>The cytoplasm in the thin portion contains <a href="/wiki/Pinocytosis" title="Pinocytosis">pinocytotic vesicles</a> which may play a role in the removal of small particulate contaminants from the outer surface. In addition to <a href="/wiki/Desmosome" title="Desmosome">desmosomes</a>, all type I alveolar cells have occluding junctions that prevent the leakage of tissue fluid into the alveolar air space. </p><p>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 <a href="/wiki/Extracellular_matrix" title="Extracellular matrix">extracellular matrix</a>, 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. </p><p>Type I pneumocytes are unable to <a href="/wiki/Mitosis" title="Mitosis">replicate</a> and are susceptible to toxic <a href="/wiki/Insult_(medical)" title="Insult (medical)">insults</a>. In the event of damage, type II cells can proliferate and differentiate into type I cells to compensate.<sup id="cite_ref-:0_18-0" class="reference"><a href="#cite_note-:0-18"><span class="cite-bracket">[</span>18<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Type_II_cells">Type II cells</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Pulmonary_alveolus&action=edit&section=6" title="Edit section: Type II cells"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Type II cells are cuboidal and much smaller than type I cells.<sup id="cite_ref-Knudsen_3-3" class="reference"><a href="#cite_note-Knudsen-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> They are the most numerous cells in the alveoli, yet do not cover as much surface area as the squamous type I cells.<sup id="cite_ref-:0_18-1" class="reference"><a href="#cite_note-:0-18"><span class="cite-bracket">[</span>18<span class="cite-bracket">]</span></a></sup> Type II cells (granulous pneumocytes) in the alveolar wall contain secretory <a href="/wiki/Organelle" title="Organelle">organelles</a> known as <a href="/wiki/Lamellar_bodies" title="Lamellar bodies">lamellar bodies</a> or lamellar granules, that fuse with the cell membranes and secrete <a href="/wiki/Pulmonary_surfactant" title="Pulmonary surfactant">pulmonary surfactant</a>. This surfactant is a film of fatty substances, a group of <a href="/wiki/Phospholipid" title="Phospholipid">phospholipids</a> that reduce alveolar <a href="/wiki/Surface_tension" title="Surface tension">surface tension</a>. The phospholipids are stored in the lamellar bodies. Without this coating, the alveoli would collapse. The surfactant is continuously released by <a href="/wiki/Exocytosis" title="Exocytosis">exocytosis</a>. Reinflation of the alveoli following exhalation is made easier by the surfactant, which reduces surface tension in the thin <a href="/wiki/Epithelial_lining_fluid" class="mw-redirect" title="Epithelial lining fluid">fluid lining of the alveoli</a>. 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 <a href="/wiki/Blood%E2%80%93air_barrier" title="Blood–air barrier">blood–air barrier</a>.<sup id="cite_ref-19" class="reference"><a href="#cite_note-19"><span class="cite-bracket">[</span>19<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-pmid11686863_20-0" class="reference"><a href="#cite_note-pmid11686863-20"><span class="cite-bracket">[</span>20<span class="cite-bracket">]</span></a></sup> </p><p>Type II cells start to develop at about 26 weeks of <a href="/wiki/Gestation" title="Gestation">gestation</a>, 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 <a href="/wiki/Infant_respiratory_distress_syndrome" title="Infant respiratory distress syndrome">infant respiratory distress syndrome</a>, which drastically reduces at ages above 35 weeks gestation. </p><p>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.<sup id="cite_ref-21" class="reference"><a href="#cite_note-21"><span class="cite-bracket">[</span>21<span class="cite-bracket">]</span></a></sup> </p><p><i><a href="/wiki/MUC1" class="mw-redirect" title="MUC1">MUC1</a></i>, a human <a href="/wiki/Gene" title="Gene">gene</a> associated with type II pneumocytes, has been identified as a marker in <a href="/wiki/Lung_cancer" title="Lung cancer">lung cancer</a>.<sup id="cite_ref-pmid9850098_22-0" class="reference"><a href="#cite_note-pmid9850098-22"><span class="cite-bracket">[</span>22<span class="cite-bracket">]</span></a></sup> </p><p>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.<sup id="cite_ref-23" class="reference"><a href="#cite_note-23"><span class="cite-bracket">[</span>23<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Alveolar_macrophages">Alveolar macrophages</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Pulmonary_alveolus&action=edit&section=7" title="Edit section: Alveolar macrophages"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The <a href="/wiki/Alveolar_macrophage" title="Alveolar macrophage">alveolar macrophages</a> 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.<sup id="cite_ref-24" class="reference"><a href="#cite_note-24"><span class="cite-bracket">[</span>24<span class="cite-bracket">]</span></a></sup> They are also called <i>pulmonary macrophages</i>, and <i>dust cells</i>. Alveolar macrophages also play a crucial role in immune responses against viral pathogens in the lungs.<sup id="cite_ref-:1_25-0" class="reference"><a href="#cite_note-:1-25"><span class="cite-bracket">[</span>25<span class="cite-bracket">]</span></a></sup> 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.<sup id="cite_ref-:1_25-1" class="reference"><a href="#cite_note-:1-25"><span class="cite-bracket">[</span>25<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Clinical_significance">Clinical significance</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Pulmonary_alveolus&action=edit&section=8" title="Edit section: Clinical significance"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <div class="mw-heading mw-heading3"><h3 id="Diseases">Diseases</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Pulmonary_alveolus&action=edit&section=9" title="Edit section: Diseases"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable">Main articles: <a href="/wiki/Respiratory_disease" title="Respiratory disease">Respiratory disease</a> and <a href="/wiki/Alveolar_lung_disease" title="Alveolar lung disease">Alveolar lung disease</a></div> <div class="mw-heading mw-heading4"><h4 id="Surfactant">Surfactant</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Pulmonary_alveolus&action=edit&section=10" title="Edit section: Surfactant"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Insufficient <a href="/wiki/Surfactant" title="Surfactant">surfactant</a> in the alveoli is one of the causes that can contribute to <a href="/wiki/Atelectasis" title="Atelectasis">atelectasis</a> (collapse of part or all of the lung). Without <a href="/wiki/Pulmonary_surfactant" title="Pulmonary surfactant">pulmonary surfactant</a>, atelectasis is a certainty.<sup id="cite_ref-26" class="reference"><a href="#cite_note-26"><span class="cite-bracket">[</span>26<span class="cite-bracket">]</span></a></sup> The severe condition of <a href="/wiki/Acute_respiratory_distress_syndrome" title="Acute respiratory distress syndrome">acute respiratory distress syndrome</a> (ARDS) is caused by a deficiency or dysfunction of surfactant.<sup id="cite_ref-Sever_27-0" class="reference"><a href="#cite_note-Sever-27"><span class="cite-bracket">[</span>27<span class="cite-bracket">]</span></a></sup> Insufficient surfactant in the lungs of preterm infants causes <a href="/wiki/Infant_respiratory_distress_syndrome" title="Infant respiratory distress syndrome">infant respiratory distress syndrome</a> (IRDS). The <a href="/wiki/Lecithin%E2%80%93sphingomyelin_ratio" title="Lecithin–sphingomyelin ratio">lecithin–sphingomyelin ratio</a> is a measure of fetal <a href="/wiki/Amniotic_fluid" title="Amniotic fluid">amniotic fluid</a> to indicate lung maturity or immaturity.<sup id="cite_ref-AJP_28-0" class="reference"><a href="#cite_note-AJP-28"><span class="cite-bracket">[</span>28<span class="cite-bracket">]</span></a></sup> A low ratio indicates a risk factor for IRDS. <a href="/wiki/Lecithin" title="Lecithin">Lecithin</a> and <a href="/wiki/Sphingomyelin" title="Sphingomyelin">sphingomyelin</a> are two of the glycolipids of pulmonary surfactant. </p><p>Impaired surfactant regulation can cause an accumulation of surfactant proteins to build up in the alveoli in a condition called <a href="/wiki/Pulmonary_alveolar_proteinosis" title="Pulmonary alveolar proteinosis">pulmonary alveolar proteinosis</a>. This results in impaired gas exchange.<sup id="cite_ref-Kumar_29-0" class="reference"><a href="#cite_note-Kumar-29"><span class="cite-bracket">[</span>29<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading4"><h4 id="Inflammation">Inflammation</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Pulmonary_alveolus&action=edit&section=11" title="Edit section: Inflammation"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p><a href="/wiki/Pneumonia" title="Pneumonia">Pneumonia</a> is an <a href="/wiki/Inflammation" title="Inflammation">inflammatory condition</a> of the <a href="/wiki/Lung_parenchyma" class="mw-redirect" title="Lung parenchyma">lung tissue</a>, which can be caused by both <a href="/wiki/Virus" title="Virus">viruses</a> and <a href="/wiki/Bacteria" title="Bacteria">bacteria</a>. <a href="/wiki/Cytokine" title="Cytokine">Cytokines</a> and fluids are released into the alveolar cavity, <a href="/wiki/Pulmonary_interstitium" class="mw-redirect" title="Pulmonary interstitium">interstitium</a>, or both, in response to infection, causing the effective surface area of gas exchange to be reduced. In severe cases where <a href="/wiki/Cellular_respiration" title="Cellular respiration">cellular respiration</a> cannot be maintained, <a href="/wiki/Supplemental_oxygen" class="mw-redirect" title="Supplemental oxygen">supplemental oxygen</a> may be required.<sup id="cite_ref-30" class="reference"><a href="#cite_note-30"><span class="cite-bracket">[</span>30<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-31" class="reference"><a href="#cite_note-31"><span class="cite-bracket">[</span>31<span class="cite-bracket">]</span></a></sup> </p> <ul><li><a href="/wiki/Diffuse_alveolar_damage" title="Diffuse alveolar damage">Diffuse alveolar damage</a> can be a cause of <a href="/wiki/Acute_respiratory_distress_syndrome" title="Acute respiratory distress syndrome">acute respiratory distress syndrome</a>(ARDS) a severe inflammatory disease of the lung.<sup id="cite_ref-DAVIDSONS2010_32-0" class="reference"><a href="#cite_note-DAVIDSONS2010-32"><span class="cite-bracket">[</span>32<span class="cite-bracket">]</span></a></sup><sup class="reference nowrap"><span title="Page / location: 187">: 187 </span></sup></li> <li>In <a href="/wiki/Asthma" title="Asthma">asthma</a>, the <a href="/wiki/Bronchiole" title="Bronchiole">bronchioles</a> become narrowed, causing the amount of air flow into the lung tissue to be greatly reduced. It can be triggered by irritants in the air, <a href="/wiki/Photochemical_smog" class="mw-redirect" title="Photochemical smog">photochemical smog</a> for example, as well as substances to which a person is allergic.</li> <li><a href="/wiki/Bronchitis#Chronic_bronchitis" title="Bronchitis">Chronic bronchitis</a> occurs when an <a href="/wiki/Mucus_hypersecretion" class="mw-redirect" title="Mucus hypersecretion">abundance of mucus</a> is produced by the lungs. The production of mucus occurs naturally when the lung tissue is exposed to irritants. In chronic bronchitis, the air passages into the alveoli, the respiratory bronchioles, become clogged with mucus. This causes increased coughing in order to remove the mucus, and is often a result of extended periods of exposure to cigarette smoke.</li> <li><a href="/wiki/Hypersensitivity_pneumonitis" title="Hypersensitivity pneumonitis">Hypersensitivity pneumonitis</a></li></ul> <div class="mw-heading mw-heading4"><h4 id="Structural">Structural</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Pulmonary_alveolus&action=edit&section=12" title="Edit section: Structural"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Cryptococcosis_of_lung_in_patient_with_AIDS._Mucicarmine_stain_962_lores.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/d/d9/Cryptococcosis_of_lung_in_patient_with_AIDS._Mucicarmine_stain_962_lores.jpg/220px-Cryptococcosis_of_lung_in_patient_with_AIDS._Mucicarmine_stain_962_lores.jpg" decoding="async" width="220" height="146" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/d/d9/Cryptococcosis_of_lung_in_patient_with_AIDS._Mucicarmine_stain_962_lores.jpg/330px-Cryptococcosis_of_lung_in_patient_with_AIDS._Mucicarmine_stain_962_lores.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/d/d9/Cryptococcosis_of_lung_in_patient_with_AIDS._Mucicarmine_stain_962_lores.jpg/440px-Cryptococcosis_of_lung_in_patient_with_AIDS._Mucicarmine_stain_962_lores.jpg 2x" data-file-width="1799" data-file-height="1196" /></a><figcaption><a href="/wiki/Cryptococcosis" title="Cryptococcosis">Cryptococcosis</a> of lung in patient with AIDS. Mucicarmine stain. Histopathology of lung shows widened alveolar septum containing a few inflammatory cells and numerous yeasts of <a href="/wiki/Cryptococcus_neoformans" title="Cryptococcus neoformans">Cryptococcus neoformans</a>. The inner layer of the yeast capsule stain red.</figcaption></figure> <p>Almost any type of <a href="/wiki/Lung_tumor" title="Lung tumor">lung tumor</a> or <a href="/wiki/Lung_cancer" title="Lung cancer">lung cancer</a> can compress the alveoli and reduce gas exchange capacity. In some cases the tumor will fill the alveoli.<sup id="cite_ref-Spencer1_33-0" class="reference"><a href="#cite_note-Spencer1-33"><span class="cite-bracket">[</span>33<span class="cite-bracket">]</span></a></sup> </p> <ul><li><a href="/wiki/Cavitary_pneumonia" class="mw-redirect" title="Cavitary pneumonia">Cavitary pneumonia</a> is a process in which the alveoli are destroyed and produce a cavity. As the alveoli are destroyed, the surface area for gas exchange to occur becomes reduced. Further changes in blood flow can lead to decline in lung function.</li> <li><a href="/wiki/Emphysema" title="Emphysema">Emphysema</a> is another disease of the lungs, whereby the <a href="/wiki/Elastin" title="Elastin">elastin</a> in the walls of the alveoli is broken down by an imbalance between the production of <a href="/wiki/Neutrophil_elastase" title="Neutrophil elastase">neutrophil elastase</a> (elevated by cigarette smoke) and <a href="/wiki/Alpha-1_antitrypsin" title="Alpha-1 antitrypsin">alpha-1 antitrypsin</a> (the activity varies due to genetics or reaction of a critical methionine residue with toxins including cigarette smoke). The resulting loss of elasticity in the lungs leads to prolonged times for exhalation, which occurs through passive recoil of the expanded lung. This leads to a smaller volume of gas exchanged per breath.</li> <li><a href="/wiki/Pulmonary_alveolar_microlithiasis" title="Pulmonary alveolar microlithiasis">Pulmonary alveolar microlithiasis</a> is a rare lung disorder of small stone formation in the alveoli.</li> <li>Several factors, including smoking, viral infections, and aging, contribute to physical damage to type II alveolar cells. Some studies have linked injury to these cells to the proliferation of <a href="/wiki/Fibrosis" title="Fibrosis">fibrosis</a> in the lungs and the onset of <a href="/wiki/Idiopathic_pulmonary_fibrosis" title="Idiopathic pulmonary fibrosis">idiopathic pulmonary fibrosis</a>.<sup id="cite_ref-34" class="reference"><a href="#cite_note-34"><span class="cite-bracket">[</span>34<span class="cite-bracket">]</span></a></sup></li></ul> <div class="mw-heading mw-heading4"><h4 id="Fluid">Fluid</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Pulmonary_alveolus&action=edit&section=13" title="Edit section: Fluid"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>A <a href="/wiki/Pulmonary_contusion" title="Pulmonary contusion">pulmonary contusion</a> is a <a href="/wiki/Bruise" title="Bruise">bruise</a> of the lung tissue caused by trauma.<sup id="cite_ref-35" class="reference"><a href="#cite_note-35"><span class="cite-bracket">[</span>35<span class="cite-bracket">]</span></a></sup> Damaged capillaries from a contusion can cause blood and other fluids to accumulate in the tissue of the lung, impairing gas exchange. </p><p><a href="/wiki/Pulmonary_edema" title="Pulmonary edema">Pulmonary edema</a> is the buildup of fluid in the parenchyma and alveoli. An edema is usually caused by left ventricular heart failure, or by damage to the lung or its vasculature. </p> <div class="mw-heading mw-heading4"><h4 id="Coronavirus">Coronavirus</h4><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Pulmonary_alveolus&action=edit&section=14" title="Edit section: Coronavirus"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable">Further information: <a href="/wiki/ACE2#Coronavirus_entry_point" class="mw-redirect" title="ACE2">ACE2 § Coronavirus entry point</a></div> <p>Because of the high expression of <a href="/wiki/Angiotensin-converting_enzyme_2" title="Angiotensin-converting enzyme 2">angiotensin-converting enzyme 2</a> (ACE2) in type II alveolar cells, the lungs are susceptible to infections by some <a href="/wiki/Coronavirus" title="Coronavirus">coronaviruses</a> including the <a href="/wiki/RNA_virus" title="RNA virus">viruses</a> that cause <a href="/wiki/Severe_acute_respiratory_syndrome" class="mw-redirect" title="Severe acute respiratory syndrome">severe acute respiratory syndrome</a> (SARS)<sup id="cite_ref-pmid20599443_36-0" class="reference"><a href="#cite_note-pmid20599443-36"><span class="cite-bracket">[</span>36<span class="cite-bracket">]</span></a></sup> and <a href="/wiki/Coronavirus_disease_2019" class="mw-redirect" title="Coronavirus disease 2019">coronavirus disease 2019</a> (COVID-19).<sup id="cite_ref-pmid32094336_37-0" class="reference"><a href="#cite_note-pmid32094336-37"><span class="cite-bracket">[</span>37<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="Additional_images">Additional images</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Pulmonary_alveolus&action=edit&section=15" title="Edit section: Additional images"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul class="gallery mw-gallery-traditional"> <li class="gallerybox" style="width: 155px"> <div class="thumb" style="width: 150px; height: 150px;"><span typeof="mw:File"><a href="/wiki/File:Pulmonary_Blood_Circulation.png" class="mw-file-description" title="Blood circulation around alveoli"><img alt="Blood circulation around alveoli" src="//upload.wikimedia.org/wikipedia/commons/thumb/2/2c/Pulmonary_Blood_Circulation.png/120px-Pulmonary_Blood_Circulation.png" decoding="async" width="120" height="73" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/2/2c/Pulmonary_Blood_Circulation.png/180px-Pulmonary_Blood_Circulation.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/2/2c/Pulmonary_Blood_Circulation.png/240px-Pulmonary_Blood_Circulation.png 2x" data-file-width="1163" data-file-height="712" /></a></span></div> <div class="gallerytext">Blood circulation around alveoli</div> </li> <li class="gallerybox" style="width: 155px"> <div class="thumb" style="width: 150px; height: 150px;"><span typeof="mw:File"><a href="/wiki/File:Plain_home_talk_about_the_human_system-the_habits_of_men_and_women-the_cause_and_prevention_of_disease-our_sexual_relations_and_social_natures_(1896)_(14577681128).jpg" class="mw-file-description" title="Diagrammatic view of lung showing magnified inner structures including alveolar sacs at 10) and lobules at 9)"><img alt="Diagrammatic view of lung showing magnified inner structures including alveolar sacs at 10) and lobules at 9)" src="//upload.wikimedia.org/wikipedia/commons/thumb/c/c0/Plain_home_talk_about_the_human_system-the_habits_of_men_and_women-the_cause_and_prevention_of_disease-our_sexual_relations_and_social_natures_%281896%29_%2814577681128%29.jpg/78px-thumbnail.jpg" decoding="async" width="78" height="120" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/c/c0/Plain_home_talk_about_the_human_system-the_habits_of_men_and_women-the_cause_and_prevention_of_disease-our_sexual_relations_and_social_natures_%281896%29_%2814577681128%29.jpg/118px-thumbnail.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/c/c0/Plain_home_talk_about_the_human_system-the_habits_of_men_and_women-the_cause_and_prevention_of_disease-our_sexual_relations_and_social_natures_%281896%29_%2814577681128%29.jpg/157px-thumbnail.jpg 2x" data-file-width="2096" data-file-height="3200" /></a></span></div> <div class="gallerytext">Diagrammatic view of lung showing magnified inner structures including alveolar sacs at 10) and lobules at 9)</div> </li> </ul> <div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Pulmonary_alveolus&action=edit&section=16" title="Edit section: See also"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a href="/wiki/Interstitial_lung_disease" title="Interstitial lung disease">Interstitial lung disease</a></li> <li><a href="/wiki/A549_cell" title="A549 cell">A549 cells</a></li> <li><a href="/wiki/List_of_distinct_cell_types_in_the_adult_human_body" class="mw-redirect" title="List of distinct cell types in the adult human body">List of distinct cell types in the adult human body</a></li></ul> <div class="mw-heading mw-heading2"><h2 id="References">References</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Pulmonary_alveolus&action=edit&section=17" title="Edit section: References"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1239543626">.mw-parser-output .reflist{margin-bottom:0.5em;list-style-type:decimal}@media screen{.mw-parser-output .reflist{font-size:90%}}.mw-parser-output .reflist .references{font-size:100%;margin-bottom:0;list-style-type:inherit}.mw-parser-output .reflist-columns-2{column-width:30em}.mw-parser-output .reflist-columns-3{column-width:25em}.mw-parser-output .reflist-columns{margin-top:0.3em}.mw-parser-output .reflist-columns ol{margin-top:0}.mw-parser-output .reflist-columns li{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .reflist-upper-alpha{list-style-type:upper-alpha}.mw-parser-output .reflist-upper-roman{list-style-type:upper-roman}.mw-parser-output .reflist-lower-alpha{list-style-type:lower-alpha}.mw-parser-output .reflist-lower-greek{list-style-type:lower-greek}.mw-parser-output .reflist-lower-roman{list-style-type:lower-roman}</style><div class="reflist"> <div class="mw-references-wrap mw-references-columns"><ol class="references"> <li id="cite_note-MeSH1-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-MeSH1_1-0">^</a></b></span> <span class="reference-text"><style data-mw-deduplicate="TemplateStyles:r1238218222">.mw-parser-output cite.citation{font-style:inherit;word-wrap:break-word}.mw-parser-output .citation q{quotes:"\"""\"""'""'"}.mw-parser-output .citation:target{background-color:rgba(0,127,255,0.133)}.mw-parser-output .id-lock-free.id-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/6/65/Lock-green.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-limited.id-lock-limited a,.mw-parser-output .id-lock-registration.id-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/d/d6/Lock-gray-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-subscription.id-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/a/aa/Lock-red-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .cs1-ws-icon a{background:url("//upload.wikimedia.org/wikipedia/commons/4/4c/Wikisource-logo.svg")right 0.1em center/12px no-repeat}body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-free a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-limited a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-registration a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-subscription a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .cs1-ws-icon a{background-size:contain;padding:0 1em 0 0}.mw-parser-output .cs1-code{color:inherit;background:inherit;border:none;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;color:var(--color-error,#d33)}.mw-parser-output .cs1-visible-error{color:var(--color-error,#d33)}.mw-parser-output .cs1-maint{display:none;color:#085;margin-left:0.3em}.mw-parser-output .cs1-kern-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right{padding-right:0.2em}.mw-parser-output .citation .mw-selflink{font-weight:inherit}@media screen{.mw-parser-output .cs1-format{font-size:95%}html.skin-theme-clientpref-night .mw-parser-output .cs1-maint{color:#18911f}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .cs1-maint{color:#18911f}}</style><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://ncbi.nlm.nih.gov/mesh/?term=pulmonary+gas+exchange">"Pulmonary Gas Exchange - MeSH"</a>. <i>ncbi.nlm.nih.gov</i>. <a href="/wiki/National_Center_for_Biotechnology_Information" title="National Center for Biotechnology Information">NCBI</a><span class="reference-accessdate">. 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class="navbox-group" style="width:1%"><a href="/wiki/Bronchus" title="Bronchus">Bronchi</a></th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li>Main <ul><li><a href="/wiki/Bronchus#Structure" title="Bronchus">left</a></li> <li><a href="/wiki/Bronchus" title="Bronchus">right</a></li></ul></li> <li><a href="/wiki/Bronchus" title="Bronchus">Secondary</a> <ul><li><a href="/wiki/Eparterial_bronchus" title="Eparterial bronchus">eparterial</a></li></ul></li> <li><a href="/wiki/Bronchus#Structure" title="Bronchus">Tertiary</a></li></ul> </div></td></tr></tbody></table><div></div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Human_lung" class="mw-redirect" title="Human lung">Lungs</a></th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"></div><table class="nowraplinks navbox-subgroup" style="border-spacing:0"><tbody><tr><th scope="row" class="navbox-group" style="width:1%">General</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li>Landmarks <ul><li><a href="/wiki/Base_of_lung" class="mw-redirect" title="Base of lung">Base</a></li> <li><a href="/wiki/Apex_of_lung" class="mw-redirect" title="Apex of lung">Apex</a></li> <li><a href="/wiki/Root_of_the_lung" title="Root of the lung">Root</a></li> <li><a href="/wiki/Hilum_of_lung" class="mw-redirect" title="Hilum of lung">Hilum</a></li></ul></li> <li>Fissures <ul><li><a href="/wiki/Horizontal_fissure_of_right_lung" class="mw-redirect" title="Horizontal fissure of right lung">Horizontal</a></li> <li><a href="/wiki/Oblique_fissure" class="mw-redirect" title="Oblique fissure">Oblique</a></li></ul></li> <li><a href="/wiki/Superior_lobe" class="mw-redirect" title="Superior lobe">Superior lobe</a></li> <li><a href="/wiki/Lingula_of_left_lung" class="mw-redirect" title="Lingula of left lung">Lingula of left lung</a></li> <li><a href="/wiki/Middle_lobe_of_right_lung" class="mw-redirect" title="Middle lobe of right lung">Middle lobe of right lung</a></li> <li><a href="/wiki/Left_lung#Lobes" class="mw-redirect" title="Left lung">Inferior lobe</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Bronchopulmonary_segment" title="Bronchopulmonary segment">BP segment</a></th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Bronchiole" title="Bronchiole">Bronchiole</a>: <a href="/wiki/Conducting_zone" class="mw-redirect" title="Conducting zone">Conducting zone</a> (<a href="/wiki/Terminal_bronchiole" class="mw-redirect" title="Terminal bronchiole">Terminal bronchiole</a>)</li> <li><a href="/wiki/Respiratory_zone" class="mw-redirect" title="Respiratory zone">Respiratory zone</a> (<a 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