View source for Bronchiole

{{short description|Passageways by which air passes through the nose or mouth to the alveoli of the lungs}}
{{More footnotes|article|date=May 2015}}
{{Infobox anatomy
| Name        = Bronchiole
| Image       = Alveoli diagram.png
| Caption     = Diagram of the [[Pulmonary alveolus|alveoli]] with both cross-section and external view.
| Image2      =
| Caption2    =
| System      = [[Respiratory system]]
| Precursor   =
}}
The '''bronchioles''' ({{IPAc-en|ˈ|b|r|ɑː|ŋ|k|i|oʊ|l|s}} {{respell|BRONG|kee|ohls}}) are the smaller branches of the [[bronchus|bronchial airways]] in the [[lower respiratory tract]]. They include the terminal bronchioles, and finally the respiratory bronchioles that mark the start of the [[respiratory zone]] delivering air to the [[gas exchange|gas exchanging units]] of the [[pulmonary alveolus|alveoli]]. The bronchioles no longer contain the cartilage that is found in the bronchi, or glands in their [[submucosa]].<ref name="TortDerr">{{cite book | vauthors = Tortora GJ |title=Principles of anatomy and physiology |date=2010 |publisher=John Wiley & Sons |location=Hoboken, NJ |isbn=9780470233474 |pages=883–888 |edition=12th}}</ref>

== Structure ==
[[File:Secondary-pulmonary-lobule-illustration.jpg|thumb|A lobule of the lung enclosed in septa and supplied by a terminal bronchiole that branches into the respiratory bronchioles. Each respiratory bronchiole supplies the alveoli held in each acinus accompanied by a pulmonary artery branch.]]
The [[pulmonary lobule]] is the portion of the [[lung]] ventilated by one bronchiole. Bronchioles are approximately 1&nbsp;mm or less in diameter and their walls consist of [[ciliated]] cuboidal [[epithelium]] and a layer of [[smooth muscle]]. 
Bronchioles divide into even smaller bronchioles, called ''terminal'', which are 0.5&nbsp;mm or less in diameter. Terminal bronchioles in turn divide into smaller respiratory bronchioles which divide into [[alveolar ducts]]. Terminal bronchioles mark the end of the conducting division of air flow in the [[respiratory system]] while respiratory bronchioles are the beginning of the respiratory division where [[gas exchange]] takes place.

The diameter of the bronchioles plays an important role in air flow. The bronchioles change diameter to either increase or reduce air flow. An increase in diameter is called [[bronchodilation]] and is stimulated by either [[epinephrine]] or [[sympathetic nerves]] to increase air flow. A decrease in diameter is called [[bronchoconstriction]], which is the tightening of the smooth muscle surrounding the bronchi and bronchioles due to and stimulated by [[histamine]], [[Parasympathetic nervous system|parasympathetic nerves]], cold air, chemical irritants, excess mucus production, viral infections, and other factors to decrease air flow. Bronchoconstriction can result in clinical symptoms such as wheezing, chest tightness, and dyspnea, which are common features of asthma, chronic obstructive pulmonary disease (COPD), and chronic bronchitis. <ref>{{cite journal | vauthors = Bacsi A, Pan L, Ba X, Boldogh I | title = Pathophysiology of bronchoconstriction: role of oxidatively damaged DNA repair | journal = Current Opinion in Allergy and Clinical Immunology | volume = 16 | issue = 1 | pages = 59–67 | date = February 2016 | pmid = 26694039 | pmc = 4940044 | doi = 10.1097/ACI.0000000000000232 }}</ref>

===Bronchioles===
[[File:Illu bronchi lungs.jpg|thumb|right|340px|Lungs showing bronchi and bronchioles]]
The [[trachea]] divides into the left main [[bronchus]] which supplies the left lung, and the right main bronchus which supplies the right lung. As they enter the lungs these primary bronchi branch into secondary bronchi known as [[lobar bronchi]] which supply each lobe of the lung. These in turn give rise to tertiary bronchi (''tertiary'' meaning "third"), known as [[segmental bronchi]] which supply each [[bronchopulmonary segment]].<ref name="TortDerr" /> The segmentary bronchi subdivide into fourth order, fifth order and sixth order segmental bronchi before dividing into the bronchioles. The bronchioles are histologically distinct from the bronchi in that their walls do not have [[hyaline cartilage]] and they have [[club cell]]s in their epithelial lining.  The [[epithelium]] of the bronchioles starts as a [[simple columnar epithelium|simple ciliated columnar epithelium]] and changes to [[simple cuboidal epithelium|simple ciliated cuboidal epithelium]] as the bronchioles decreases in size. The diameter of the bronchioles is often said to be less than 1&nbsp;mm, though this value can range from 5&nbsp;mm to 0.3&nbsp;mm. As stated, these bronchioles do not have hyaline cartilage to maintain their patency. Instead, they rely on [[elastic fiber]]s attached to the surrounding [[human lung|lung]] tissue for support.  The inner lining ([[lamina propria]]) of these bronchioles is thin with no glands present, and is surrounded by a layer of [[smooth muscle]].  As the bronchioles get smaller they divide into terminal bronchioles. Each bronchiole divides into between 50 and 80 terminal bronchioles.<ref name="KS">{{cite book | vauthors = Saladin K |title=Human anatomy |date=2011 |publisher=McGraw-Hill |isbn=9780071222075 |pages=640–641 |edition=3rd}}</ref> These bronchioles mark the end of the [[conducting zone]], which covers the first division through the sixteenth division of the [[respiratory tract]]. [[Pulmonary alveolus|Alveoli]] only become present when the conducting zone changes to the [[respiratory zone]], from the sixteenth through the twenty-third division of the tract.

===Terminal bronchioles===
The terminal bronchioles are the most distal segment of the conducting zone. They branch off the lesser bronchioles. Each of the terminal bronchioles divides to form respiratory bronchioles which contain a small number of alveoli.  Terminal bronchioles are lined with [[simple cuboidal epithelium|simple ciliated cuboidal epithelium]] containing [[club cell]]s. Club cells are non-ciliated, rounded protein-secreting cells.  Their secretions are a non-sticky, proteinaceous compound to maintain the airway in the smallest bronchioles.  The secretion, called [[pulmonary surfactant]], reduces [[surface tension]], allowing for bronchioles to expand during inspiration and keeping the bronchioles from collapsing during expiration.  Club cells are a [[stem cell]] of the [[respiratory system]], and also produce [[enzyme]]s that detoxify substances dissolved in the respiratory fluid.

===Respiratory bronchioles===
The respiratory bronchioles are the narrowest airways of the lungs, 0.5 mm across.<ref name="Merck">{{cite book |title=Merck Manual of Medical Information |date=1997 |publisher=Merck Research Laboratories |location=Whitehouse Station, N.J. |isbn=978-0-911910-87-2 |edition=Home}}</ref> The [[bronchi]] divide many times before evolving into the bronchioles.
The respiratory bronchioles deliver air to the exchange surfaces of the lungs.<ref>{{cite book | vauthors = Martini FH, Timmons MJ, Tallitsch RB |title=Human Anatomy |publisher=Benjamin Cummings |isbn=978-0-321-49804-5 |page=643 |edition=6th}}</ref>
They are interrupted by [[Pulmonary alveolus|alveoli]] which are thin walled [[evagination]]s. [[Alveolar ducts]] are side branches of the respiratory bronchioles. The respiratory bronchioles are lined by ciliated cuboidal epithelium along with some non-ciliated cells called [[club cells]].<ref> {{cite web | title = Respiratory: Trachea, bronchioles and bronchi | year = 2003 | publisher = University of Leeds | url = http://www.histology.leeds.ac.uk/respiratory/conducting.php | last1 = Paxton | first1 = Steve | last2 = Peckham | first2 = Michelle | last3 = Knibbs | first3 = Adele }}</ref>

==Clinical significance==
[[Bronchospasm]], a potentially life-threatening situation, occurs when the smooth muscular tissue of the bronchioles constricts, severely narrowing their diameter. The most common cause of this is asthma. Bronchospasm is commonly treated by [[oxygen therapy]] and [[bronchodilator]]s such as [[albuterol]].

Diseases of the bronchioles include [[asthma]], [[bronchiolitis obliterans]], [[respiratory syncytial virus]] infections, and [[influenza]].

===Inflammation===
The medical condition of inflammation of the bronchioles is termed [[bronchiolitis]].<ref name=Fri2014>{{cite journal | vauthors = Friedman JN, Rieder MJ, Walton JM | title = Bronchiolitis: Recommendations for diagnosis, monitoring and management of children one to 24 months of age | journal = Paediatrics & Child Health | volume = 19 | issue = 9 | pages = 485–498 | date = November 2014 | pmid = 25414585 | pmc = 4235450 | doi = 10.1093/pch/19.9.485 }}</ref>

==Additional images==
<gallery>
 Image:Tertiary bronchus.JPG|Cross sectional cut of primary bronchiole
 Image:illu_quiz_lung05.jpg |{{ordered list |[[Vertebrate trachea|Trachea]] |Primary [[bronchus]] |[[Secondary bronchus|Lobar bronchus]] |[[Tertiary bronchus|Segmental bronchus]] |Bronchiole |[[Alveolar duct]] |[[Pulmonary alveolus|Alveolus]]}}
</gallery>

== References ==
{{Reflist}}

== Further reading ==
* Saladin, Kenneth S. Anatomy & Physiology: the Unity of Form and Function. New York, NY: McGraw-Hill, 2007.
* Dudek, Ronald W. ''High-Yield Histology'', 3rd ed. (2004). {{ISBN|0-7817-4763-5}}
* Gartner, Leslie P. and James L. Hiatt. ''Color Atlas of Histology'', 3rd ed. (2000). {{ISBN|0-7817-3509-2}}
* Gartner, Leslie P. and James L. Hiatt. ''Color Textbook of Histology'' (2001). {{ISBN|0-7216-8806-3}}

== External links ==
* {{BUHistology|13606loa}}
* {{BUHistology|13607loa}}
* [http://arquivo.pt/wayback/20160523193246/http://www.bio.davidson.edu/people/midorcas/animalphysiology/Human%20Physiology%20Manual/chapter4/exper10/back_chap4_exp10.htm Diagram at davidson.edu]
* [https://web.archive.org/web/20080307173126/http://www3.umdnj.edu/histsweb/lab10/lab10lung.html Histology at umdnj.edu]

{{Lower respiratory system anatomy}}
{{Authority control}}

[[Category:Lung anatomy]]