Editing Respiratory system (section)

===Anatomy===
{{Main|Lung|Respiratory tract}}
[[File:Poumons2.jpg|thumb|left|300px|'''Fig. 1.''' Respiratory system]]
[[File:illu quiz lung05.jpg|thumb|180px|'''Fig. 2.''' The [[Respiratory tract#Lower respiratory tract|lower respiratory tract]], or "Respiratory Tree"{{ordered list |[[Vertebrate trachea|Trachea]] |[[Main bronchus|Mainstem bronchus]] |[[Secondary bronchus|Lobar bronchus]] |[[Tertiary bronchus|Segmental bronchus]] |[[Bronchiole]] |[[Alveolar duct]] |[[Pulmonary alveolus|Alveolus]]}}]]
In [[human]]s and other [[mammal]]s, the anatomy of a typical respiratory system is the [[respiratory tract]]. The tract is divided into an [[Respiratory tract#Upper respiratory tract|upper]] and a [[Respiratory tract#Lower respiratory tract|lower respiratory tract]]. The upper tract includes the [[nose]], [[nasal cavity|nasal cavities]], [[paranasal sinuses|sinuses]], [[pharynx]] and the part of the [[larynx]] above the [[vocal folds]]. The lower tract (Fig.&nbsp;2.) includes the lower part of the [[larynx]], the [[trachea]], [[bronchus|bronchi]], [[bronchiole]]s and the [[pulmonary alveolus|alveoli]].

The branching airways of the lower tract are often described as the [[Respiratory tract#Lower respiratory tract|respiratory tree]] or [[tracheobronchial tree]] (Fig.&nbsp;2).<ref name=gilroy>{{cite book|last1=Gilroy|first1=Anne M.|last2=MacPherson|first2= Brian R.|last3= Ross|first3=Lawrence M.|title= Atlas of Anatomy|publisher=Thieme|location=Stuttgart|date=2008|pages=108–111|isbn=978-1-60406-062-1}}</ref> The intervals between successive branch points along the various branches of "tree" are often referred to as branching "generations", of which there are, in the adult human, about 23. The earlier generations (approximately generations 0–16), consisting of the trachea and the bronchi, as well as the larger bronchioles which simply act as [[Conducting zone|air conduits]], bringing air to the respiratory bronchioles, alveolar ducts and alveoli (approximately generations 17–23), where [[gas exchange]] takes place.<ref name="Pocock">{{cite book|last1=Pocock|first1=Gillian|last2=Richards|first2=Christopher D.|title=Human physiology : the basis of medicine|date=2006|publisher=Oxford University Press|location=Oxford|isbn=978-0-19-856878-0|pages=315–317|edition=3rd}}</ref><ref name=tortora1 /> [[Bronchiole]]s are defined as the small airways lacking any [[cartilaginous]] support.<ref name=gilroy />

The first bronchi to branch from the [[trachea]] are the right and left main bronchi. Second, only in diameter to the trachea (1.8&nbsp;cm), these bronchi (1–1.4&nbsp;cm in diameter)<ref name="Pocock"/> enter the [[lung]]s at each [[Root of the lung|hilum]], where they branch into narrower secondary bronchi known as lobar bronchi, and these branch into narrower tertiary bronchi known as segmental bronchi. Further divisions of the segmental bronchi (1 to 6&nbsp;mm in diameter)<ref name="Kacmarek">{{cite book|last1=Kacmarek|first1=Robert M.|last2=Dimas|first2=Steven|last3=Mack|first3=Craig W.|title=Essentials of Respiratory Care - E-Book|url=https://books.google.com/books?id=FV9PAQAAQBAJ&pg=PA81|publisher=Elsevier Health Sciences|language=en|date=13 August 2013|isbn=9780323277785}}</ref> 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, US|url=https://archive.org/details/humanbiologyheal00scho|isbn=0-13-981176-1}}{{Page needed|date=September 2010}}</ref>

Compared to the 23 number (on average) of branchings of the respiratory tree in the adult human, the [[mouse]] has only about 13 such branchings.

The alveoli are the dead end terminals of the "tree", meaning that any air that enters them has to exit via the same route. A system such as this creates [[Dead space (physiology)|dead space]], a volume of air (about 150&nbsp;ml in the adult human) that fills the airways after exhalation and is breathed back into the alveoli before environmental air reaches them.<ref name=fowler1948 /><ref>{{cite web|title=anatomical dead space|url=http://medical-dictionary.thefreedictionary.com/anatomical+dead+space|website=TheFreeDictionary.com}}</ref> At the end of inhalation, the airways are filled with environmental air, which is exhaled without coming in contact with the gas exchanger.<ref name=fowler1948 />