View source for Bronchus

{{short description|Airway in the respiratory tract}}
{{Infobox anatomy
| Name        = Bronchus
| Latin       = bronchus
| Greek      = βρόγχος
| Image       = 3D Medical Animation Bronchus Description.jpg
| Caption     = The bronchi are conducting passages for air into the lungs.
| Image2      = Blausen 0770 RespiratorySystem 02.png
| Caption2    = The bronchi form part of the [[lower respiratory tract]]
| Precursor   =
| System      = [[Respiratory system]]
| Artery      = [[Bronchial artery]]
| Vein        = [[Bronchial vein]]
| Nerve       = [[Pulmonary branches of vagus nerve]]
| Lymph       =
}}
A '''bronchus''' ({{IPAc-en|ˈ|b|ɹ|ɒ|ŋ|k|ə|s}} {{respell|BRONG|kəs}}; {{plural form}}: '''bronchi''', {{IPAc-en|ˈ|b|ɹ|ɒ|ŋ|k|aɪ}} {{respell|BRONG|ky}}) is a passage or airway in the [[lower respiratory tract]] that conducts [[Atmosphere of Earth|air]] into the [[lung]]s. The first or primary bronchi to branch from the [[trachea]] at the [[Carina of trachea|carina]] are the right main bronchus and the left main bronchus. These are the widest bronchi, and enter the right lung, and the left lung at each [[Root of the lung|hilum]]. The main bronchi branch into narrower secondary bronchi or lobar bronchi, and these branch into narrower tertiary bronchi or segmental bronchi. Further divisions of the segmental bronchi are known as 4th order, 5th order, and 6th order segmental bronchi, or grouped together as subsegmental bronchi.<ref name="Netter">{{cite book|last1=Netter|first1=Frank H.|title=Atlas of Human Anatomy Including Student Consult Interactive Ancillaries and Guides.|date=2014|publisher=W B Saunders Co|location=Philadelphia, Penn.|isbn=978-1-4557-0418-7|page=200|edition=6th}}</ref><ref>{{Cite book|last=Maton|first=Anthea|author2=Jean Hopkins|author3=Charles William McLaughlin|author4=Susan Johnson|author5=Maryanna Quon Warner|author6=David LaHart|author7=Jill D. Wright|title=Human Biology and Health|publisher=Prentice Hall|year=1993|location=wood Cliffs, New Jersey, USA|url=https://archive.org/details/humanbiologyheal00scho|isbn=0-13-981176-1|url-access=registration}}{{Page needed|date=September 2010}}</ref>
The bronchi, when too narrow to be supported by cartilage, are known as [[bronchiole]]s. No [[gas exchange]] takes place in the bronchi.

==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&nbsp;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&nbsp;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&nbsp;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]].

==Function==
{{See also|Respiratory tract#Lower respiratory tract}}

The bronchi function to carry [[air]] that is breathed in through to the functional tissues of the lungs, called alveoli. Exchange of gases between the air in the lungs and the blood in the [[capillaries]] occurs across the walls of the alveolar ducts and alveoli. The [[alveolar ducts]] and alveoli consist primarily of simple [[squamous epithelium]], which permits rapid diffusion of [[oxygen]] and [[carbon dioxide]].

==Clinical significance==
[[File:Bronchial wall thickness (T) and diameter (D).svg|thumb|100px|Bronchial wall thickness (T) and bronchial diameter (D).]]
Bronchial wall thickening, as can be seen on [[CT scan]], generally (but not always) implies [[inflammation]] of the bronchi ([[bronchitis]]).<ref>{{cite web |author=Weerakkody |first=Yuranga |date=2021-01-13 |title=Bronchial wall thickening |url=https://radiopaedia.org/articles/bronchial-wall-thickening |access-date=2018-01-05 |website=[[Radiopaedia]]}}</ref> Normally, the ratio of the bronchial wall thickness and the bronchial diameter is between 0.17 and 0.23.<ref>[https://books.google.com/books?id=97pHzCI5vE4C&pg=PA112 Page 112] in: {{cite book|title=Imaging of the Airways: Functional and Radiologic Correlations|author=David P. Naidich|publisher=Lippincott Williams & Wilkins|year=2005|isbn=9780781757683}}</ref>

===Bronchitis===
{{Main|Bronchitis}}
Bronchitis is defined as inflammation of the bronchi, which can either be [[Acute (medicine)|acute]] or [[chronic condition|chronic]]. [[Acute bronchitis]] is usually caused by [[virus disease|viral]] or [[pathogenic bacteria|bacterial infections]]. Many sufferers of [[chronic bronchitis]] also suffer from [[chronic obstructive pulmonary disease]] (COPD), and this is usually associated with [[smoking]] or long-term exposure to [[irritation|irritants]].

===Aspiration===
The left main bronchus departs from the trachea at a greater angle than that of the right main bronchus. The right bronchus is also wider than the left and these differences predispose the right lung to [[pulmonary aspiration|aspirational]] problems. If food, liquids, or foreign bodies are aspirated, they will tend to lodge in the right main bronchus. [[Bacterial pneumonia]] and [[aspiration pneumonia]] may result.

If a [[tracheal tube]] used for [[tracheal intubation|intubation]] is inserted too far, it will usually lodge in the right bronchus, allowing ventilation only of the right lung.

===Asthma===
[[Asthma]] is marked by [[bronchial hyperresponsiveness|hyperresponsiveness of the bronchi]] with an inflammatory component, often in response to [[allergen]]s.

In asthma, the constriction of the bronchi can result in difficulty in breathing giving [[shortness of breath]]; this can lead to a [[Hypoxia (medical)|lack of oxygen]] reaching the body for cellular processes. In this case, an [[inhaler]] can be used to rectify the problem. The inhaler administers a [[bronchodilator]], which serves to soothe the constricted bronchi and to re-expand the airways. This effect occurs quite quickly.

===Bronchial atresia===
'''Bronchial atresia''' is a rare congenital disorder that can have a varied appearance. A bronchial atresia is a defect in the development of the bronchi, affecting one or more bronchi – usually segmental bronchi and sometimes lobar. The defect takes the form of a blind-ended bronchus. The surrounding [[lung parenchyma|tissue]] secretes mucus normally but builds up and becomes distended.<ref name="JVS">{{cite journal|pmid=29302450|pmc=5730535|year=2017|last1=Traibi|first1=A.|title=Congenital bronchial atresia in adults: Thoracoscopic resection|journal=Journal of Visualized Surgery|volume=3|pages=174|last2=Seguin-Givelet|first2=A.|last3=Grigoroiu|first3=M.|last4=Brian|first4=E.|last5=Gossot|first5=D.|doi=10.21037/jovs.2017.10.15 |doi-access=free }}</ref> This can lead to [[emphysema|regional emphysema]].<ref name="Thorax">{{cite journal|pmid=1494776|pmc=1021111|year=1992|last1=Van Klaveren|first1=R. J.|title=Congenital bronchial atresia with regional emphysema associated with pectus excavatum|journal=Thorax|volume=47|issue=12|pages=1082–3|last2=Morshuis|first2=W. J.|last3=Lacquet|first3=L. K.|last4=Cox|first4=A. L.|last5=Festen|first5=J.|last6=Heystraten|first6=F. M.|doi=10.1136/thx.47.12.1082}}</ref>

The collected mucus may form a mucoid impaction or a [[bronchocele]], or both. A [[pectus excavatum]] may accompany a bronchial atresia.<ref name="JVS"/>

==Additional images==
<gallery>
 File:Bronchi.JPG|Cross-section of secondary bronchus
 File:Relations of the aorta, trachea, esophagus and other heart structures.png|The left and right main bronchi sit behind the heart, shown here.
</gallery>

== Citations ==
{{Reflist|2}}

==Sources==
* Moore, Keith L. and Arthur F. Dalley. ''Clinically Oriented Anatomy, 4th ed.'' (1999). {{ISBN|0-7817-5936-6}}.

==External links==

{{System and organs}}
{{Respiratory system}}
{{Authority control}}

[[Category:Bronchus| ]]
[[Category:Respiratory system anatomy]]
[[Category:Thorax (human anatomy)]]