Editing Pulmonary alveolus (section)

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

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

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

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

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

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