Editing Lung (section)

====Alveoli====
{{Main|Pulmonary alveolus}}
[[File:Alveolus diagram.svg|thumb|right|Alveoli and their capillary networks]]
[[File:3D Medical Animation Bronchial Airways terminating ends.jpg|alt=A 3D Medical illustration showing different terminating ends of Bronchial airways connected to alveoili, lung parenchyma & lymphatic vessels.|thumb|3D medical illustration showing different terminating ends of bronchioles]]
Alveoli consist of two types of [[alveolar cell]] and an [[alveolar macrophage]]. The two types of cell are known as [[Pulmonary alveolus#Type I cells|type I]] and [[Pulmonary alveolus#Type II cells|type II cells]]<ref name=BERNELEVY /> (also known as pneumocytes).<ref name=GRAYS40TH /> Types I and II make up the walls and [[alveolar septum|alveolar septa]]. Type I cells provide 95% of the surface area of each alveoli and are flat ("[[squamous epithelial cell|squamous]]"), and Type II cells generally cluster in the corners of the alveoli and have a cuboidal shape.<ref name=PAWLINA /> Despite this, cells occur in a roughly equal ratio of 1:1 or 6:4.<ref name=BERNELEVY /><ref name=PAWLINA />

Type I are [[epithelium|squamous epithelial cells]] that make up the alveolar wall structure. They have extremely thin walls that enable an easy gas exchange.<ref name=BERNELEVY /> These type I cells also make up the alveolar septa which separate each alveolus. The septa consist of an epithelial lining and associated [[basement membrane]]s.<ref name=PAWLINA /> Type I cells are not able to divide, and consequently rely on [[cellular differentiation|differentiation]] from Type II cells.<ref name=PAWLINA />

Type II are larger and they line the alveoli and produce and secrete epithelial lining fluid, and [[pulmonary surfactant|lung surfactant]].<ref name=pmid26475269>{{cite journal |last1=Srikanth |first1=Lokanathan |last2=Venkatesh |first2=Katari |last3=Sunitha |first3=Manne Mudhu |last4=Kumar |first4=Pasupuleti Santhosh |last5=Chandrasekhar |first5=Chodimella |last6=Vengamma |first6=Bhuma |last7=Sarma |first7=Potukuchi Venkata Gurunadha Krishna |title=In vitro generation of type-II pneumocytes can be initiated in human CD34+ stem cells |journal=Biotechnology Letters |date=16 October 2015 |volume=38 |issue=2 |pages=237–242 |doi=10.1007/s10529-015-1974-2 |pmid=26475269 |s2cid=17083137 }}</ref><ref name=BERNELEVY /> Type II cells are able to divide and differentiate to Type I cells.<ref name=PAWLINA />

The [[alveolar macrophage]]s have an important role in the [[immune system]]. They remove substances which deposit in the alveoli including loose red blood cells that have been forced out from blood vessels.<ref name=PAWLINA />