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Bronchus
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==Structure== The [[trachea]] (windpipe) divides at the [[carina of trachea|carina]] into two main or '''primary bronchi''', the left bronchus and the right bronchus. The carina of the trachea is located at the level of the [[sternal angle]] and the fifth [[thoracic vertebra]] (at rest). The '''right main bronchus''' is wider, shorter, and more vertical than the left main bronchus,<ref name=Brodsky2003>{{cite journal|last1=Brodsky|first1=JB|last2=Lemmens|first2=JM|title=Left Double-Lumen Tubes: Clinical Experience With 1,170 Patients|journal=Journal of Cardiothoracic and Vascular Anesthesia|volume=17|issue=3|pages=289–98|year=2003|pmid=12827573|doi=10.1016/S1053-0770(03)00046-6|url=http://ether.stanford.edu/library/thoracic_anesthesia/One-Lung%20Ventilation/DLT%20Placement.pdf|archive-url=https://web.archive.org/web/20170312234953/http://ether.stanford.edu/library/thoracic_anesthesia/One-Lung%20Ventilation/DLT%20Placement.pdf|archive-date=2017-03-12}} [https://zenodo.org/record/896726 Alt URL]</ref> its mean length is 1.09 cm.<ref name="Robinson">{{cite journal |last1=Robinson |first1=CL |last2=Müller |first2=NL |last3=Essery |first3=C |title=Clinical significance and measurement of the length of the right main bronchus. |journal=Canadian Journal of Surgery |date=January 1989 |volume=32 |issue=1 |pages=27–8 |pmid=2642720}}</ref> It enters the [[root of the lung|root of the right lung]] at approximately the fifth thoracic vertebra. The right main bronchus subdivides into three '''secondary bronchi''' (also known as '''lobar bronchi'''), which deliver oxygen to the three [[Human lung#Lobes|lobes]] of the right lung—the superior, middle and inferior lobe. The [[azygos vein]] arches over it from behind; and the right pulmonary artery lies at first below and then in front of it. About 2 cm from its commencement it gives off a branch to the superior lobe of the right lung, which is also called the [[eparterial bronchus]]. ''Eparterial'' refers to its position above the right [[pulmonary artery]]. The right bronchus now passes below the artery, and is known as the ''hyparterial'' branch which divides into the two lobar bronchi to the middle and lower lobes. The '''left main bronchus''' is smaller in caliber but longer than the right, being 5 cm long. It enters the [[root of the lung|root of the left lung]] opposite the sixth thoracic vertebra. It passes beneath the [[aortic arch]], crosses in front of the [[esophagus]], the [[thoracic duct]], and the [[descending aorta]], and has the left pulmonary artery lying at first above, and then in front of it. The left bronchus has no [[eparterial branch]], and therefore it has been supposed by some that there is no upper lobe to the left lung, but that the so-called upper lobe corresponds to the middle lobe of the [[right lung]]. The left main bronchus divides into two secondary bronchi or lobar bronchi, to deliver air to the two lobes of the left lung—the superior and the inferior lobe. {{Anchor|Segmental bronchus|Subsegmental bronchus}}The secondary bronchi divide further into '''tertiary bronchi''', (also known as '''segmental bronchi'''), each of which supplies a [[bronchopulmonary segment]]. A bronchopulmonary segment is a division of a lung separated from the rest of the lung by a [[septum]] of [[connective tissue]]. This property allows a bronchopulmonary segment to be surgically removed without affecting other segments. Initially, there are ten segments in each lung, but during development with the left lung having just two lobes, two pairs of segments fuse to give eight, four for each lobe. The tertiary bronchi divide further in another three branchings known as 4th order, 5th order and 6th order segmental bronchi which are also referred to as '''subsegmental bronchi'''. These branch into many smaller [[bronchiole]]s which divide into [[terminal bronchiole]]s, each of which then gives rise to several [[respiratory bronchiole]]s, which go on to divide into two to eleven [[alveolar duct]]s. There are five or six [[alveolar sac]]s associated with each alveolar duct. The [[pulmonary alveolus|alveolus]] is the basic anatomical unit of gas exchange in the lung. The main bronchi have relatively large [[lumen (anatomy)|lumens]] that are lined by [[respiratory epithelium]]. This cellular lining has cilia departing towards the mouth which removes dust and other small particles. There is a [[smooth muscle]] layer below the epithelium arranged as two ribbons of muscle that spiral in opposite directions. This smooth muscle layer contains [[seromucous gland]]s, which secrete [[mucus]], in its wall. [[Hyaline cartilage]] is present in the bronchi, surrounding the smooth muscle layer. In the main bronchi, the cartilage forms C-shaped rings like those in the trachea, while in the smaller bronchi, hyaline cartilage is present in irregularly arranged crescent-shaped plates and islands. These plates give structural support to the bronchi and keep the airway open.<ref name="Saladin2">{{cite book |last1=Saladin |first1=K |title=Anatomy & physiology : the unity of form and function |date=2012 |publisher=McGraw-Hill |isbn=9780073378251 |page=862 |edition=6th}}</ref> The bronchial wall normally has a thickness of 10% to 20% of the total bronchial diameter.<ref>[https://books.google.com/books?id=OlhuhvvR7z4C&pg=SA6-PA4 Section SA6-PA4 ("Airway Inflammation with Wall Thickening"]) in: {{cite book|title=Fundamentals of High-Resolution Lung CT: Common Findings, Common Patterns, Common Diseases, and Differential Diagnosis|author=Brett M. Elicker, W. Richard Webb|publisher=Lippincott Williams & Wilkins|year=2012|isbn=9781469824796}}</ref> ===Microanatomy=== {{Main|Respiratory epithelium}} [[File:Bronchiolar epithelium 1 - SEM.jpg|thumb|[[Cilium|Cilia]] and much smaller [[Microvillus|microvilli]] on non-ciliated bronchiolar epithelium]] The [[cartilage]] and [[mucous membrane]] of the main bronchus (primary bronchi) are similar to those in the trachea. They are lined with [[respiratory epithelium]], which is classified as [[Pseudostratified columnar epithelium|ciliated pseudostratified columnar epithelium]].<ref name="marieb"/> The epithelium in the main bronchi contains [[goblet cell]]s, which are [[gland]]ular, modified simple [[columnar epithelium|columnar epithelial cells]] that produce [[mucins]], the main component of mucus. Mucus plays an important role in keeping the airways clear in the [[mucociliary clearance]] process. As branching continues through the bronchial tree, the amount of hyaline cartilage in the walls decreases until it is absent in the bronchioles. As the cartilage decreases, the amount of smooth muscle increases. The mucous membrane also undergoes a transition from ciliated pseudostratified columnar epithelium, to simple ciliated cuboidal epithelium, to simple [[squamous epithelium]] in the alveolar ducts and alveoli<ref name="marieb"> {{cite book | last1 = Marieb| first1 = Elaine N. | first2 = Katja| last2 = Hoehn | title =Human Anatomy & Physiology | publisher =Pearson | edition =9th | date =2012 | isbn = 978-0321852120 }}</ref><ref name=NIH>{{Cite web|url=https://www.nih.gov|title=Bronchi, Bronchial Tree & Lungs|website=nih.gov|access-date=18 September 2019}}</ref> ===Variation=== In 0.1 to 5% of people there is a right superior lobe bronchus arising from the main stem bronchus prior to the carina. This is known as a '''tracheal bronchus''', and seen as an [[anatomical variation]].<ref name="Weerakkody">{{cite web |last1=Weerakkody |first1=Yuranga |title=Tracheal bronchus {{!}} Radiology Reference Article {{!}} Radiopaedia.org |url=https://radiopaedia.org/articles/tracheal-bronchus?lang=gb |website=Radiopaedia |access-date=21 November 2021}}</ref> It can have multiple variations and, although usually asymptomatic, it can be the root cause of pulmonary disease such as a recurrent infection. In such cases [[segmental resection|resection]] is often curative.<ref>{{Cite journal | doi = 10.1503/cmaj.080280 | pmid = 19332762 | issn = 0820-3946 | volume = 180 | issue = 7 | pages = 783 | last = Shih | first = Fu-Chieh |author2=Wei-Jing Lee |author3=Hung-Jung Lin | title = Tracheal bronchus | journal = Canadian Medical Association Journal | date = 2009-03-31 | pmc = 2659830 }} </ref> <ref> {{Cite journal | doi = 10.1016/S0196-0709(87)80034-0 | pmid = 3592078 | issn = 0196-0709 | volume = 8 | issue = 2 | pages = 118–122 | last = Barat | first = Michael |author2=Horst R. Konrad | title = Tracheal bronchus | journal = American Journal of Otolaryngology | date = 1987-03-04 }}</ref> The '''cardiac bronchus''' has a prevalence of ≈0.3% and presents as an accessory bronchus arising from the bronchus intermedius between the upper lobar bronchus and the origin of the middle and lower lobar bronchi of the right main bronchus.<ref>{{cite web |title=Cardiac bronchus |work=Radiopedia |url=http://radiopaedia.org/articles/cardiac-bronchus |archive-url=https://web.archive.org/web/20151115054026/http://radiopaedia.org/articles/cardiac-bronchus |archive-date=2015-11-15 |url-status=live }}</ref> An accessory cardiac bronchus is usually an asymptomatic condition but may be associated with persistent infection or [[hemoptysis]].<ref>Parker MS, Christenson ML, Abbott GF. ''Teaching atlas of chest imaging.'' 2006, {{ISBN|3131390212}}</ref><ref>{{cite journal |vauthors=McGuinness G, Naidich DP, Garay SM, Davis AL, Boyd AD, Mizrachi HH |title=Accessory cardiac bronchus: CT features and clinical significance |journal=Radiology |volume=189 |issue=2 |pages=563–6 |year=1993 |pmid=8210391 |doi=10.1148/radiology.189.2.8210391 }}</ref> In about half of observed cases the cardiac bronchus presents as a short dead-ending bronchial stump, in the remainder the bronchus may exhibit branching and associated aerated [[lung parenchyma]].
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