Editing Gill (section)

==Function==
Many microscopic aquatic animals, and some larger but inactive ones, can absorb sufficient oxygen through the entire surface of their bodies, and so can respire adequately without gills. However, more complex or more active aquatic organisms usually require one gill or more. Many invertebrates, and even amphibians, use both the body surface and gills for gaseous exchange.<ref name=Dorit>{{cite book |title=Zoology |url=https://archive.org/details/zoology0000dori |url-access=registration |last1=Dorit |first1=R. L. |last2=Walker |first2=W. F. |last3=Barnes |first3=R. D. |year=1991 |publisher=Saunders College Publishing |isbn=978-0-03-030504-7 |pages=[https://archive.org/details/zoology0000dori/page/273 273–276] }}</ref>

Gills usually consist of thin filaments of [[tissue (biology)|tissue]], [[Lamella (surface anatomy)|lamellae]] (plates), branches, or slender, tufted [[Process (anatomy)|processes]] that have a highly folded surface to increase [[surface area]].{{clarify |reason=sentence is confusing to read |date=March 2025}} The delicate nature of the gills is possible because the surrounding water provides support. The blood or other body fluid must be in intimate contact with the respiratory surface for ease of diffusion.<ref name=Dorit/>

A high surface area is crucial to the [[gas exchange]] of aquatic organisms, as water contains only a small fraction of the [[dissolved oxygen]] than [[air]] does, and it diffuses more slowly. A [[cubic meter]] of air contains about 275 [[gram]]s of oxygen at [[Standard temperature and pressure|STP]]. [[Fresh water]] hold less than 1/25th the oxygen content of air, the dissolved oxygen content being approximately 8&nbsp;cm<sup>3</sup>/L compared to that of air which is 210&nbsp;cm<sup>3</sup>/L.<ref name="Advanced Biology">{{cite book|title=Advanced Biology|author1=M. b. v. Roberts |author2=Michael Reiss |author3=Grace Monger |pages=164–165|publisher=Nelson|year=2000|location=London, UK}}</ref> Water is 777 times more dense than air and is 100 times more viscous.<ref name="Advanced Biology"/> Oxygen has a diffusion rate in air 10,000 times greater than in water.<ref name="Advanced Biology"/> The use of sac-like lungs to remove oxygen from water would not be efficient enough to sustain life.<ref name="Advanced Biology"/> Rather than using lungs, "[g]aseous exchange takes place across the surface of highly vascularised gills over which a one-way current of water is kept flowing by a specialised pumping mechanism. The density of the water prevents the gills from collapsing and lying on top of each other; [such collapse] happens when a fish is taken out of water."<ref name="Advanced Biology"/>

Usually water is moved across the gills in one direction by the current, by the motion of the animal through the water, by the beating of cilia or other appendages, or by means of a pumping mechanism. In fish and some molluscs, the efficiency of the gills is greatly enhanced by a [[countercurrent exchange]] mechanism in which the water passes over the gills in the opposite direction to the flow of blood through them. This mechanism is very efficient and as much as 90% of the dissolved oxygen in the water may be recovered.<ref name=Dorit/>