Editing Pulmonary alveolus (section)

== Structure ==
[[File:Bronchial anatomy with description.png|thumb|upright=1.4|[[Bronchiole|Bronchial]] anatomy showing [[terminal bronchioles]] (BT) leading to [[respiratory bronchiole]]s (BR) and alveolar ducts (DA) that open into alveolar sacs containing out pockets of alveoli (A) separated by [[alveolar septum|alveolar septa]] (AS)]]
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 '''alveolar ducts''' that become deeply lined with alveoli. The ducts number between two and eleven from each bronchiole.<ref name="Spencer 2">{{cite book |title=Spencer's pathology of the lung |date=1996 |publisher=McGraw-Hill |location=New York |isbn=0-07-105448-0 |pages=22–25 |edition=5th}}</ref> Each duct opens into five or six '''alveolar sacs''' into which clusters of alveoli open.

Each terminal respiratory unit is called an [[Lung#Respiratory zone|acinus]] and consists of the respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli. New alveoli continue to form until the age of eight years.<ref name="Moore">{{cite book |title=Clinically oriented anatomy |last=Moore|first=Keith | name-list-style = vanc |publisher=Wolters Kluwer |year=2018 |isbn=978-1-4963-4721-3 |page=336}}</ref>

A typical pair of [[Lung|human lungs]] contains about 480 million alveoli,<ref name="pmid14512270">{{cite journal | vauthors = Ochs M | title = The number of alveoli in the human lung | journal = Am J Respir Crit Care Med | volume = 1 | issue = 169 | pages = 120–4 | date = 2004 | pmid = 14512270 | doi = 10.1164/rccm.200308-1107OC}}</ref> providing a total surface area for gas exchange of between 70 and 80 square metres.<ref name="Spencer 2"/> Each alveolus is wrapped in a fine mesh of [[capillaries]] covering about 70% of its area.<ref name="Saladin"/> The diameter of an alveolus is between 200 and 500 [[micrometre|μm]].<ref name="Saladin">Stanton, Bruce M.; Koeppen, Bruce A., eds. (2008). Berne & Levy physiology (6th ed.). Philadelphia: Mosby/Elsevier. pp. 418–422. {{ISBN|978-0-323-04582-7}}.</ref>

=== Microanatomy ===
An alveolus consists of an [[epithelial]] layer of simple [[Epithelium#Cell types|squamous epithelium]] (very thin, flattened cells),<ref name=NIH>{{cite web |url= https://training.seer.cancer.gov/anatomy/respiratory/passages/bronchi.html |title=Bronchi, Bronchial Tree & Lungs | work = SEER Training Modules | publisher = U.S. Department of Health and Human Services National Institutes of Health National Cancer Institute}}</ref> and an [[extracellular matrix]] surrounded by [[Capillary|capillaries]]. The epithelial lining is part of the alveolar membrane, also known as the respiratory membrane, that allows the [[gas exchange|exchange of gases]]. The membrane has several layers – a layer of [[alveolar lining fluid]] that contains [[surfactant]], the epithelial layer and its basement membrane; a thin [[interstitium|interstitial space]] between the epithelial lining and the capillary membrane; a capillary basement membrane that often fuses with the alveolar basement membrane, and the capillary [[endothelial]] membrane. The whole membrane however is only between 0.2 [[micrometre|μm]] at its thinnest part and 0.6 μm at its thickest.<ref name="Hall">{{Cite book|title=Guyton and Hall Textbook of Medical Physiology|last=Hall|first=John | name-list-style = vanc |publisher=Saunders Elsevier|year=2011|isbn=978-1-4160-4574-8|pages=489–491}}</ref>

In the '''alveolar walls''' there are interconnecting air passages between the alveoli known as the [[pores of Kohn]]. The '''alveolar septum''' that separates the alveoli in the alveolar sac contains some [[collagen |collagen fibers]] and [[elastic fibers]]. The septa also house the enmeshed capillary network that surrounds each alveolus.<ref name="Knudsen"/> 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.
[[File:Alveolar sac.JPG|thumb|A histologic slide of a human alveolar sac]]

There are three major types of '''alveolar cell'''. Two types are '''pneumocytes''' or '''pneumonocytes''' known as type I and type II cells found in the alveolar wall, and a large [[phagocyte|phagocytic cell]] known as an [[alveolar macrophage]] 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 [[pulmonary surfactant]] to lower [[surface tension]], and can also [[cellular differentiation|differentiate]] to replace damaged type I cells.<ref name="Saladin"/><ref name="Naeem">{{cite journal |last1=Naeem |first1=Ahmed |last2=Rai |first2=Sachchida N. |last3=Pierre |first3=Louisdon |title=Histology, Alveolar Macrophages |url=https://www.ncbi.nlm.nih.gov/books/NBK513313/ |website=StatPearls |publisher=StatPearls Publishing |access-date=12 September 2021 |date=2021|pmid=30020685}}</ref>