Editing Gas exchange (section)

==Summary of main gas exchange systems==

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!  !! Surface area !! Diffusion distance !! Maintaining concentration gradient !! Respiratory organs
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| '''Human''' || Total alveoli<ref name="Basset1987">{{Cite journal|vauthors=Basset J, Crone C, Saumon G | title=Significance of active ion transport in transalveolar water absorption: a study on isolated rat lung| journal=[[The Journal of Physiology]]| volume=384| year=1987| pages=311–324| doi=10.1113/jphysiol.1987.sp016456| pmid=3656149| pmc=1192264}}</ref> = 70–100&nbsp;m<sup>2</sup>|| Alveolus and capillary (two cells)|| Constant blood flow in capillaries; breathing || Lungs
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| '''Fish''' || Many lamellae and filaments per gill || Usually one cell || Countercurrent flow || Gills
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| '''Insects''' || Specialised tracheole cell || One cell || Buccal pumping || Spiracles
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| '''Sponges''' || Ostia pores || One cell || Water movement || None
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| '''Flatworms''' || Flat body shape || Usually one cell || Countercurrent flow || None
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| '''Cnidarians''' || Oral arms || Usually one cell || Water movement || None
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| '''Reptiles''' || Many lamellae and filaments per gill{{clarify|reason=what gills?|date=April 2020}} || Alveolus and capillary (two cells) || Countercurrent flow || Lungs
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| '''Amphibians''' || Many lamellae and filaments per gill || Alveolus and capillary (two cells) or one cell || Countercurrent flow || Lungs, skin and gills
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| '''Plants''' || High density of stomata; air spaces within leaf || One cell || Constant air flow || Stomata
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