Editing Lung (section)

=== Birds ===
{{Main|Bird anatomy#Respiratory system}}
[[File:BirdRespiration.svg|thumb|left|On inhalation, air travels to air sacs near the back of a bird. The air then passes through the lungs to air sacs near the front of the bird, from where the air is exhaled.]] [[File:Cross-current exchanger.jpg|thumb|right|The cross-current respiratory gas exchanger in the lungs of birds. Air is forced from the air sacs unidirectionally (from left to right 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=15|pages=2455–2462|date=2011|doi=10.1242/jeb.052548|pmid=21753038|doi-access=free}}</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.]]
The lungs of birds are relatively small, but are connected to eight or nine [[air sacs]] that extend through much of the body, and are in turn connected to air spaces within the bones. On inhalation, air travels through the trachea of a bird into the air sacs. Air then travels continuously from the air sacs at the back, through the lungs, which are relatively fixed in size, to the air sacs at the front. From here, the air is exhaled. These fixed size lungs are called "circulatory lungs", as distinct from the "bellows-type lungs" found in most other animals.<ref name="AvResp"/><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>

The lungs of birds contain millions of tiny parallel passages called [[parabronchi]]. Small sacs called ''atria'' radiate from the walls of the tiny passages; these, like the alveoli in other lungs, are the site of [[gas exchange]] by simple diffusion.<ref name=Maina2005 /> The blood flow around the parabronchi and their atria forms a cross-current process of gas exchange (see diagram on the right).<ref name=AvResp /><ref name= graham />

The air sacs, which hold air, do not contribute much to gas exchange, despite being thin-walled, as they are poorly vascularised. The air sacs expand and contract due to changes in the volume in the thorax and abdomen. This volume change is caused by the movement of the sternum and ribs and this movement is often synchronised with movement of the flight muscles.<ref name=VB>{{Cite book|last1=Romer |first1=Alfred Sherwood |last2=Parsons |first2=Thomas S. |year=1977 |title=The Vertebrate Body |publisher=Holt-Saunders International |location=Philadelphia |pages=330–334 |isbn=978-0-03-910284-5}}</ref>

Parabronchi in which the air flow is unidirectional are called ''paleopulmonic parabronchi'' and are found in all birds. Some birds, however, have, in addition, a lung structure where the air flow in the parabronchi is bidirectional. These are termed ''neopulmonic parabronchi''.<ref name=Maina2005 />