Editing Respiratory system (section)

==Birds==
{{See also|Bird anatomy#Respiratory system}}

[[File:Cranial_sinus_and_postcranial_air_sac_systems_in_birds.svg|thumb|left|250 px| '''Fig. 15''' The arrangement of the air sacs and lungs in birds]]
[[File:Bird's respiratory system.jpg|thumb|right|250 px|'''Fig. 16''' The anatomy of bird's respiratory system, showing the relationships of the trachea, primary and intra-pulmonary bronchi, the dorso- and ventro-bronchi, with the parabronchi running between the two. The posterior and anterior air sacs are also indicated, but not to scale.]]
[[File:Inhalation in birds.jpg|thumb|left|250 px|'''Fig. 17''' A [[dove]] skeleton, showing the movement of the chest during inhalation. '''Arrow 1''' indicates the movement of the vertebral ribs. '''Arrow 2''' shows the consequent movement of the [[sternum]] (and its [[Keel (bird)|keel]]). The two movements increase the vertical and transverse diameters of the chest portion of the trunk of the bird.<br /> Key:<br />
1. [[skull]]; 2. [[cervical vertebrae]]; 3. [[furcula]]; 4. [[coracoid]]; 5. '''vertebral ribs'''; 6. '''sternum and its keel'''; 7. [[patella]]; 8. [[tarsometatarsus|tarsus]]; 9. [[Digit (anatomy)|digits]]; 10. [[tibia]] ([[tibiotarsus]]); 11. [[fibula]] ([[tibiotarsus]]); 12. [[femur]]; 13. [[ischium]] ([[Hip bone|innominate]]); 14. [[pubis (bone)|pubis]] (innominate); 15. [[Ilium (bone)|ilium]] (innominate); 16. [[caudal vertebrae]]; 17. [[pygostyle]]; 18. [[synsacrum]]; 19. [[scapula]]; 20. [[dorsal vertebrae]]; 21. [[humerus]]; 22. [[ulna]]; 23. [[Radius (bone)|radius]]; 24. [[Carpal bones|carpus]] ([[carpometacarpus]]); 25. [[metacarpus]] ([[carpometacarpus]]); 26. [[Digit (anatomy)|digits]]; 27. [[alula]]]]

The respiratory system of birds differs significantly from that found in mammals. Firstly, they have rigid lungs which do not expand and contract during the breathing cycle. Instead an extensive system of [[air sacs]] (Fig.&nbsp;15) distributed throughout their bodies act as the bellows drawing environmental air into the sacs, and expelling the spent air after it has passed through the lungs (Fig.&nbsp;18).<ref name="campbell">{{cite book|last1=Campbell|first1=Neil A.|title=Biology|date=1990|publisher=Benjamin/Cummings Pub. Co.|location=Redwood City, Calif.|isbn=0-8053-1800-3|pages=836–844|edition=2nd}}</ref> Birds also do not have [[Thoracic diaphragm|diaphragms]] or [[Pleural cavity|pleural cavities]].

Bird lungs are smaller than those in mammals of comparable size, but the air sacs account for 15% of the total body volume, compared to the 7% devoted to the [[Pulmonary alveolus|alveoli]] which act as the bellows in mammals.<ref name="Whittow 2000 233–241" />

Inhalation and exhalation are brought about by alternately increasing and decreasing the volume of the entire thoraco-abdominal cavity (or [[Body cavity#Coelom|coelom]]) using both their abdominal and costal muscles.<ref name=AvResp /><ref name=storer3>{{cite book|last1=Storer|first1=Tracy I.|last2=Usinger|first2=R. L.|last3=Stebbins|first3=Robert C.|last4=Nybakken|first4=James W.|title=General Zoology|edition=sixth|publisher=McGraw-Hill|location=New York|date=1997|pages=[https://archive.org/details/generalzoolog00stor/page/752 752–753]|isbn=0-07-061780-5|url=https://archive.org/details/generalzoolog00stor/page/752}}</ref><ref name=Romer>{{cite book|last=Romer|first=Alfred Sherwood|title=The Vertebrate body|edition=Fourth|publisher=W.B. Saunders|location=Philadelphia|date=1970|pages=[https://archive.org/details/vertebratebody00holt/page/323 323–324]|isbn=0-7216-7667-7|url=https://archive.org/details/vertebratebody00holt/page/323}}</ref> During inhalation the muscles attached to the vertebral ribs (Fig.&nbsp;17) contract angling them forwards and outwards. This pushes the sternal ribs, to which they are attached at almost right angles, downwards and forwards, taking the [[sternum]] (with its prominent [[Keel (bird anatomy)|keel]]) in the same direction (Fig.&nbsp;17). This increases both the vertical and transverse diameters of thoracic portion of the trunk. The forward and downward movement of, particularly, the [[Anatomical terms of location#Main terminologies|posterior]] end of the sternum pulls the abdominal wall downwards, increasing the volume of that region of the trunk as well.<ref name=AvResp /> The increase in volume of the entire trunk cavity reduces the air pressure in all the thoraco-abdominal air sacs, causing them to fill with air as described below.

During exhalation the external oblique muscle which is attached to the sternum and vertebral ribs  [[Anatomical terms of location#Main terminologies|anteriorly]], and to the pelvis (pubis and ilium in Fig.&nbsp;17) [[Anatomical terms of location#Main terminologies|posteriorly]] (forming part of the abdominal wall) reverses the inhalatory movement, while compressing the abdominal contents, thus increasing the pressure in all the air sacs. Air is therefore expelled from the respiratory system in the act of exhalation.<ref name=AvResp />

[[File:Cross-current exchanger.jpg|thumb|right|250 px|'''Fig. 19''' The [[Countercurrent exchange|cross-current]] respiratory gas exchanger in the lungs of birds. Air is forced from the air sacs [[Unidirectional respiratory system|unidirectionally]] (from right to left in the diagram) through the parabronchi. The pulmonary capillaries surround the parabronchi in the manner shown (blood flowing from below the parabronchus to above it in the diagram).<ref name=AvResp>{{cite web| url = http://www.people.eku.edu/ritchisong/birdrespiration.html | title = BIO 554/754 – Ornithology: Avian respiration | access-date = 2009-04-23 | last = Ritchson | first = G | publisher = Department of Biological Sciences, Eastern Kentucky University }}</ref><ref name= graham>{{cite journal|last=Scott|first=Graham R.|title=Commentary: Elevated performance: the unique physiology of birds that fly at high altitudes|journal=Journal of Experimental Biology|volume= 214|issue=Pt 15|pages=2455–2462|date=2011|doi=10.1242/jeb.052548|pmid=21753038|doi-access=|s2cid=27550864 }}</ref> Blood or air with a high oxygen content is shown in red; oxygen-poor air or blood is shown in various shades of purple-blue.]]

During inhalation air enters the [[Vertebrate trachea|trachea]] via the nostrils and mouth, and continues to just beyond the [[syrinx (bird anatomy)|syrinx]] at which point the trachea branches into two [[Bronchus|primary bronchi]], going to the two lungs (Fig.&nbsp;16).  The primary bronchi enter the lungs to become the intrapulmonary bronchi, which give off a set of parallel branches called ventrobronchi and, a little further on, an equivalent set of dorsobronchi (Fig.&nbsp;16).<ref name=AvResp /> The ends of the intrapulmonary bronchi discharge air into the posterior air sacs at the [[Anatomical terms of location#Caudal|caudal]] end of the bird. Each pair of  dorso-ventrobronchi is connected by a large number of parallel microscopic air capillaries (or [[parabronchi]]) where [[gas exchange]] occurs (Fig.&nbsp;16).<ref name=AvResp /> As the bird inhales, tracheal air flows through the intrapulmonary bronchi into the posterior air sacs, as well as into the ''dorso''bronchi, but not into the ''ventro''bronchi (Fig.&nbsp;18). This is due to the bronchial architecture which directs the inhaled air away from the openings of the ventrobronchi, into the continuation of the intrapulmonary bronchus towards the dorsobronchi and posterior air sacs.<ref name="Maina2005">{{cite book|last1=Maina|first1=John N.|title=The lung air sac system of birds development, structure, and function; with 6 tables|date=2005|publisher=Springer|location=Berlin|isbn=978-3-540-25595-6|pages=3.2–3.3 "Lung", "Airway (Bronchiol) System" 66–82|url=https://books.google.com/books?id=-wtoEg7fcjkC&q=neopulmonic+parabronchi&pg=PA66}}</ref><ref name="Krautwald-Junghanns, et al. 2010">{{cite book|last=Krautwald-Junghanns|first=Maria-Elisabeth|title=Diagnostic Imaging of Exotic Pets: Birds, Small Mammals, Reptiles|year=2010|publisher=Manson Publishing|location=Germany|isbn=978-3-89993-049-8|display-authors=etal}}</ref><ref name=sturkie>{{cite book|last=Sturkie |first= P.D. |editor1-first= P. D |editor1-last= Sturkie |title=Avian Physiology | publisher=Springer Verlag |location= New York |date= 1976 |page = 201 |isbn= 978-1-4612-9335-4 |doi= 10.1007/978-1-4612-4862-0|s2cid= 36415426 }}</ref> From the dorsobronchi the inhaled air flows through the parabronchi (and therefore the gas exchanger) to the ventrobronchi from where the air can only escape into the expanding anterior air sacs. So, during inhalation, both the posterior and anterior air sacs expand,<ref name=AvResp /> the posterior air sacs filling with fresh inhaled air, while the anterior air sacs fill with "spent" (oxygen-poor) air that has just passed through the lungs.

[[File:BirdRespiration.svg|thumb|right|250 px|'''Fig. 18''' Inhalation-exhalation cycle in birds]]
During exhalation the pressure in the posterior air sacs (which were filled with fresh air during inhalation) increases due to the contraction of the oblique muscle described above. The aerodynamics of the interconnecting openings from the posterior air sacs to the dorsobronchi and intrapulmonary bronchi ensures that the air leaves these sacs in the direction of the lungs (via the dorsobronchi), rather than returning down the intrapulmonary bronchi (Fig.&nbsp;18).<ref name="Maina2005" /><ref name=sturkie /> From the dorsobronchi the fresh air from the posterior air sacs flows through the parabronchi (in the same direction as occurred during inhalation) into ventrobronchi. The air passages connecting the ventrobronchi and anterior air sacs to the intrapulmonary bronchi direct the "spent", oxygen poor air from these two organs to the trachea from where it escapes to the exterior.<ref name=AvResp /> Oxygenated air therefore flows constantly (during the entire breathing cycle) in a single direction through the parabronchi.<ref>Ritchison, Gary. "Ornithology (Bio 554/754):Bird Respiratory System". Eastern Kentucky University. Retrieved 2007-06-27.</ref>

The blood flow through the bird lung is at right angles to the flow of air through the parabronchi, forming a cross-current flow exchange system (Fig.&nbsp;19).<ref name=campbell /><ref name="AvResp"/><ref name="graham"/> The [[Blood gas tension|partial pressure of oxygen]] in the parabronchi declines along their lengths as O<sub>2</sub> diffuses into the blood. The blood capillaries leaving the exchanger near the entrance of airflow take up more O<sub>2</sub> than do the capillaries leaving near the exit end of the parabronchi.  When the contents of all capillaries mix, the final partial pressure of oxygen of the mixed pulmonary venous blood is higher than that of the exhaled air,<ref name=AvResp /><ref name= graham /> but is nevertheless less than half that of the inhaled air,<ref name=AvResp /> thus achieving roughly the same systemic arterial blood partial pressure of oxygen as [[#Gas exchange|mammals do with their bellows-type lungs]].<ref name=AvResp />

The trachea is an area of [[Dead space (physiology)|dead space]]: the oxygen-poor air it contains at the end of exhalation is the first air to re-enter the posterior air sacs and lungs. In comparison to the [[#Anatomy|mammalian respiratory tract]], the dead space volume in a bird is, on average, 4.5&nbsp;times greater than it is in mammals of the same size.<ref name="Whittow 2000 233–241">{{cite book|last=Whittow|first=G. Causey|title=Sturkie's Avian Physiology|year=2000|publisher=Academic Press|location=San Diego, California|isbn=978-0-12-747605-6 |pages=233–241}}</ref><ref name=AvResp /> Birds with long necks will inevitably have long tracheae, and must therefore take deeper breaths than mammals do to make allowances for their greater dead space volumes. In some birds (e.g. the [[whooper swan]], ''Cygnus cygnus'', the [[white spoonbill]], ''Platalea leucorodia'', the [[whooping crane]], ''Grus americana'', and the [[helmeted curassow]], ''Pauxi pauxi'') the trachea, which some cranes can be 1.5&nbsp;m long,<ref name=AvResp /> is coiled back and forth within the body, drastically increasing the dead space ventilation.<ref name=AvResp /> The purpose of this extraordinary feature is unknown.