Editing Pulmonary alveolus (section)

=== 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>