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| Bronchiole | |
|---|---|
 Diagram of thealveoli with both cross-section and external view. | |
| Details | |
| System | Respiratory system |
| Identifiers | |
| MeSH | D055745 |
| TA98 | A06.5.02.026 |
| TA2 | 3282 |
| TH | H3.05.02.0.00005 |
| FMA | 7410 |
| Anatomical terminology | |
Thebronchioles (/ˈbrɑːŋkioʊls/BRONG-kee-ohls) are the smaller branches of thebronchial airways in thelower respiratory tract. They include the terminal bronchioles, and finally the respiratory bronchioles that mark the start of therespiratory zone delivering air to thegas exchanging units of thealveoli. The bronchioles no longer contain the cartilage that is found in the bronchi, or glands in theirsubmucosa.[1]
Thepulmonary lobule is the portion of thelung ventilated by one bronchiole. Bronchioles are approximately 1 mm or less in diameter and their walls consist ofciliated cuboidalepithelium and a layer ofsmooth muscle. Bronchioles divide into even smaller bronchioles, calledterminal, which are 0.5 mm or less in diameter. Terminal bronchioles in turn divide into smaller respiratory bronchioles which divide intoalveolar ducts. Terminal bronchioles mark the end of the conducting division of air flow in therespiratory system while respiratory bronchioles are the beginning of the respiratory division wheregas 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 calledbronchodilation and is stimulated by eitherepinephrine orsympathetic nerves to increase air flow. A decrease in diameter is calledbronchoconstriction, which is the tightening of the smooth muscle surrounding the bronchi and bronchioles due to and stimulated byhistamine,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.[2]
Thetrachea divides into the left mainbronchus 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 aslobar bronchi which supply each lobe of the lung. These in turn give rise to tertiary bronchi (tertiary meaning "third"), known assegmental bronchi which supply eachbronchopulmonary segment.[1] 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 havehyaline cartilage and they haveclub cells in their epithelial lining. Theepithelium of the bronchioles starts as asimple ciliated columnar epithelium and changes tosimple ciliated cuboidal epithelium as the bronchioles decreases in size. The diameter of the bronchioles is often said to be less than 1 mm, though this value can range from 5 mm to 0.3 mm. As stated, these bronchioles do not have hyaline cartilage to maintain their patency. Instead, they rely onelastic fibers attached to the surroundinglung tissue for support. The inner lining (lamina propria) of these bronchioles is thin with no glands present, and is surrounded by a layer ofsmooth muscle. As the bronchioles get smaller they divide into terminal bronchioles. Each bronchiole divides into between 50 and 80 terminal bronchioles.[3] These bronchioles mark the end of theconducting zone, which covers the first division through the sixteenth division of therespiratory tract.Alveoli only become present when the conducting zone changes to therespiratory zone, from the sixteenth through the twenty-third division of the tract.
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 withsimple ciliated cuboidal epithelium containingclub cells. 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, calledpulmonary surfactant, reducessurface tension, allowing for bronchioles to expand during inspiration and keeping the bronchioles from collapsing during expiration. Club cells are astem cell of therespiratory system, and also produceenzymes that detoxify substances dissolved in the respiratory fluid.
The respiratory bronchioles are the narrowest airways of the lungs, 0.5 mm across.[4] Thebronchi divide many times before evolving into the bronchioles.The respiratory bronchioles deliver air to the exchange surfaces of the lungs.[5]They are interrupted byalveoli which are thin walledevaginations.Alveolar ducts are side branches of the respiratory bronchioles. The respiratory bronchioles are lined by ciliated cuboidal epithelium along with some non-ciliated cells calledclub cells.[6]
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 byoxygen therapy andbronchodilators such asalbuterol.
Diseases of the bronchioles includeasthma,bronchiolitis obliterans,respiratory syncytial virus infections, andinfluenza.
The medical condition of inflammation of the bronchioles is termedbronchiolitis.[7]
