Editing Trachea (section)

== Structure ==
[[File:Blausen 0865 TracheaAnatomy.png|thumb]]

An adult's trachea has an inner diameter of about {{convert|1.5|to|2|cm|frac=4}} and a length of about {{convert|10|to|11|cm|frac=4|abbr=on}}, wider in males than females.<ref name=GA2016>{{Cite book|title=Gray's anatomy : the anatomical basis of clinical practice| veditors = Standring S |isbn=9780702052309|edition=41st|location=Philadelphia|oclc=920806541|year=2016|pages=965–969|section=Trachea and bronchi| last1 = Standring | first1 = Susan }}</ref> The trachea begins at the lower edge of the [[cricoid cartilage]] of the larynx<ref name="Furlow2018">{{cite journal |vauthors=Furlow PW, Mathisen DJ |date=March 2018 |title=Surgical anatomy of the trachea |journal=Annals of Cardiothoracic Surgery |volume=7 |issue=2 |pages=255–260 |doi=10.21037/acs.2018.03.01 |pmc=5900092 |pmid=29707503 |doi-access=free}}</ref> at the level of sixth [[cervical vertebra]] (C6)<ref name=GA2016 /> and ends at the carina, the point where the trachea branches into left and right main bronchi.,<ref name="GA2016" /> at the level of the fourth [[thoracic vertebra]] (T4),<ref name=GA2016 /> although its position may change with [[breathing]].<ref name="Furlow2018" /> The trachea is surrounded by 16–20 rings of [[hyaline cartilage]]; these 'rings' are 4 millimetres high in the adult, incomplete and C-shaped.<ref name="GA2016" /> Ligaments connect the rings.<ref name="Furlow2018" /> The [[trachealis muscle]] connects the ends of the incomplete rings and runs along the back wall of the trachea.<ref name="Furlow2018" /> Also adventitia, which is the outermost layer of connective tissue that surrounds the hyaline cartilage, contributes to the trachea's ability to bend and stretch with movement.<ref>{{cite journal | vauthors = Ohkimoto K, Mouri M, Amatsu M, Teraoka M | title = [Histological study of the tracheal adventitia, perichondrium and annular ligament] | journal = Nihon Jibiinkoka Gakkai Kaiho | volume = 100 | issue = 11 | pages = 1394–1400 | date = November 1997 | pmid = 9423323 | doi = 10.3950/jibiinkoka.100.1394 | doi-access = free }}</ref>

Although trachea is a midline structure, it can be  displaced normally to the right by the aortic arch.<ref name="pmid8462036">{{cite journal | vauthors = Dennie CJ, Coblentz CL | title = The trachea: normal anatomic features, imaging and causes of displacement | journal = Canadian Association of Radiologists Journal | volume = 44 | issue = 2 | pages = 81–9 | date = April 1993 | pmid = 8462036 | doi = | url = }}</ref>

=== Nearby structures ===
The trachea passes by many structures of the [[neck]] and chest ([[thorax]]) along its course.

In front of the upper trachea lies connective tissue and skin.<ref name="GA2016" /> Several other structures pass over or sit on the trachea; the [[Jugular venous arch|jugular arch]], which joins the two [[anterior jugular vein]]s, sits in front of the upper part of the trachea. The [[Sternohyoid muscle|sternohyoid]] and [[sternothyroid muscle]]s stretch along its length. The thyroid gland also stretches across the upper trachea, with the isthmus overlying the second to fourth rings, and the lobes stretching to the level of the fifth or sixth cartilage.<ref name="GA2016" /> The blood vessels of the thyroid rest on the trachea next to the isthmus; [[superior thyroid arteries]] join just above it, and the [[inferior thyroid veins]] below it.<ref name="GA2016" /> In front of the lower trachea lies the manubrium of the [[sternum]], the [[Thymus#Involution|remnants]] of the [[thymus]] in adults. To the front left lie the large blood vessels the [[aortic arch]] and its branches the left [[common carotid artery]] and the [[Brachiocephalic artery|brachiocephalic trunk]]; and the left [[brachiocephalic vein]]. The deep [[cardiac plexus]] and lymph nodes are also positioned in front of the lower trachea.<ref name="GA2016" />

Behind the trachea, along its length, sits the [[Esophagus|oesophagus]], followed by connective tissue and the [[vertebral column]].<ref name=GA2016 /> To its sides run the [[carotid arteries]] and inferior thyroid arteries; and to its sides on its back surface run the [[recurrent laryngeal nerve]]s in the upper trachea, and the [[vagus nerve]]s in the lower trachea.<ref name=GA2016 />

The trachealis muscle contracts during [[cough]]ing, reducing the size of the [[lumen (anatomy)|lumen]] of the trachea.<ref name="Furlow2018" />

<gallery>
File:CT-Thorax-5.0-B70f.ogg|CT scan of the thorax (axial lung window)
File:COR-2-STND-CHEST.ogg|CT scan of the thorax (coronal lung window) 
File:COR-2-STND-CHEST-MEDIASTINUM.ogg|CT scan of the thorax (coronal mediastinal window) 
File:Cross section of a trachea and esophagus.svg|Cross section of a trachea and esophagus
File:Sternohyoid muscle.PNG|The [[sternohyoid muscle|sternohyoid]] and [[sternothyroid muscle]]s lie on top of the upper part of the trachea
File:Blausen 0534 Goiter.png|The thyroid gland also lies on top of the trachea, and lies below the cricoid cartilage. 
</gallery>

===Blood and lymphatic supply===
[[File:Gray622.png|thumb|[[Lymph node]]s of the trachea]]

The upper part of trachea receives and drains [[blood]] through the [[inferior thyroid arteries]] and veins;<ref name=GA2016 /> the lower trachea receives blood from bronchial arteries.<ref name="Furlow2018" /> Arteries that supply the trachea do so via small branches that supply the trachea from the sides. As the branches approach the wall of the trachea, they split into [[Anatomical terms of location#Superior and inferior|inferior and superior]] branches, which join with the branches of the arteries above and below; these then split into branches that supply the anterior and posterior parts of the trachea.<ref name="Furlow2018" /> The inferior thyroid arteries arise just below the isthmus of the thyroid, which sits atop the trachea. These arteries join ({{wt|en|anastamoses}}) with ascending branches of the [[bronchial artery|bronchial arteries]], which are direct branches from the [[aorta]], to supply blood to the trachea.<ref name=GA2016 /> The [[lymphatic vessel]]s of the trachea drain into the [[pretracheal lymph nodes|pretracheal nodes]] that lie in front of the trachea, and [[paratracheal lymph nodes]] that lie beside it.<ref name=GA2016 />

===Development===
In the fourth week of [[human embryogenesis|development of the human embryo]] as the [[respiratory bud]] grows, the trachea separates from the [[foregut]] through the formation of ridges which eventually separate the trachea from the oesophagus, the [[tracheoesophageal septum]]. This separates the future trachea from the oesophagus and divides the foregut tube into the laryngotracheal tube.<ref name=Langmans2019 >{{cite book | vauthors = Sadley TW |title=Langman's medical embryology |date=2019|location=Philadelphia|publisher=Wolters Kluwer |isbn=9781496383907 |edition=14th|pages=223–229}}</ref> By the start of the fifth week, the left and right main bronchi have begun to form, initially as buds at the terminal end of the trachea.<ref name=Langmans2019 />

The trachea is no more than 4&nbsp;mm in diameter during the first year of life, expanding to its adult diameter of approximately 2&nbsp;cm by late childhood.<ref name=GA2016 /><ref name="Furlow2018" /> The trachea is more circular and more vertical in children compared to adults,<ref name="Furlow2018" /> varies more in size, and also varies more in its position in relation to its surrounding structures.<ref name="GA2016" />

===Microanatomy===
The trachea is lined with a layer of [[pseudostratified ciliated columnar epithelium|interspersed layers of column-shaped cells with cilia]].<ref name="Furlow2018" /> The [[epithelium]] contains [[goblet cell]]s,  which are [[gland]]ular, column-shaped cells that produce [[mucins]], the main component of [[mucus]]. Mucus helps to moisten and protect the airways.<ref>{{cite book | vauthors = Mescher AL | chapter = Chapter 17. The Respiratory System | veditors = Mescher AL | title = Junqueira's Basic Histology: Text & Atlas | date = 28 August 2009 | publisher = McGraw Hill Professional | edition = 12th | url = http://www.accessmedicine.com/content.aspx?aID=6182422 |access-date=2015-02-24 |url-status=dead |archive-url=https://web.archive.org/web/20130603013209/http://www.accessmedicine.com/content.aspx?aid=6182422 |archive-date=3 June 2013 | isbn = 978-0-07-163020-7 }}</ref> Mucus lines the [[ciliated]] cells of the trachea to trap inhaled foreign particles that the cilia then waft upward toward the larynx and then the pharynx where it can be either swallowed into the [[stomach]] or expelled as [[phlegm]]. This self-clearing mechanism is termed [[mucociliary clearance]].<ref>{{cite journal | vauthors = Antunes MB, Cohen NA | title = Mucociliary clearance--a critical upper airway host defense mechanism and methods of assessment | journal = Current Opinion in Allergy and Clinical Immunology | volume = 7 | issue = 1 | pages = 5–10 | date = February 2007 | pmid = 17218804 | doi = 10.1097/aci.0b013e3280114eef | s2cid = 9551913 }}</ref> Directly beneath this mucus layer lies the submucosa layer which is composed primarily of fibrous connective tissue and connects the mucosa to the rings of hyaline cartilage beneath.<ref>Ayyalasomayajula, Venkat, and Bjørn Skallerud. “Microstructure and Mechanics of the Bovine Trachea: Layer Specific Investigations through SHG Imaging and Biaxial Testing.” Journal of the Mechanical Behavior of Biomedical Materials, vol. 134, 1 Oct. 2022, p. 5, https://doi.org/10.1016/j.jmbbm.2022.105371.</ref>

The trachea is surrounded by 16 to 20 rings of hyaline cartilage; these 'rings' are incomplete and C-shaped.<ref name=GA2016 /> Two or more of the cartilages often unite, partially or completely, and they are sometimes bifurcated at their extremities. The rings are generally highly elastic but they may [[calcification|calcify]] with [[ageing|age]].
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<gallery>
File:Gray964.png|Cross-section
File:2308 The Trachea-b.jpg|Cross-section of the trachea, with pseudostratified ciliated columnar epithelium and goblet cells labelled
File:Cartilage01.JPG|Magnified cross-section of the cartilage of the trachea 
</gallery>