Editing Gas exchange (section)

===Fish===
[[File:Tuna Gills in Situ 01.jpg|300 px|thumb|left|'''Fig. 8.''' Gills of tuna showing filaments and lamellae]]

The dissolved oxygen content in [[fresh water]] is approximately 8–10&nbsp;milliliters per liter compared to that of air which is 210&nbsp;milliliters per liter.<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 800 times more dense than air<ref name=tyson>{{cite book|last1=Tyson|first1=P. D.|last2=Preston-White|first2=R.A.|title=The Weather and Climate of Southern Africa|edition=Second|date=2013|publisher=Oxford University Press| location=Cape Town, South Africa|page=14|isbn=9780195718065}}</ref> and 100 times more viscous.<ref name="Advanced Biology"/> Therefore, 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 therefore not be efficient enough to sustain life.<ref name="Advanced Biology"/> Rather than using lungs, gaseous exchange takes place across the surface of highly vascularized [[Fish gill|gill]]s. Gills are specialised organs containing [[Gill filament|filaments]], which further divide into [[lamella (anatomy)|lamellae]]. The lamellae contain [[capillaries]] that provide a large surface area and short diffusion distances, as their walls are extremely thin.<ref name="Newstead1967">{{Cite journal| author=Newstead James D | title=Fine structure of the respiratory lamellae of teleostean gills| journal=[[Cell and Tissue Research]]| volume=79| issue=3| year=1967| pages=396–428| doi=10.1007/bf00335484| pmid=5598734| s2cid=20771899}}</ref> Gill rakers are found within the exchange system in order to filter out food, and keep the gills clean.

Gills use a [[countercurrent flow]] system that increases the efficiency of oxygen-uptake (and waste gas loss).<ref name=campbell /><ref name="Hughes1972" /><ref name=storer/> Oxygenated water is drawn in through the mouth and passes over the gills in one direction while blood flows through the lamellae in the opposite direction. This [[countercurrent exchange|countercurrent]] maintains steep concentration gradients along the entire length of each capillary (see the diagram in the [[#Interaction with circulatory systems|"Interaction with circulatory systems"]] section above). Oxygen is able to continually diffuse down its gradient into the blood, and the carbon dioxide down its gradient into the water.<ref name="Hughes1972"/> The deoxygenated water will eventually pass out through the [[Operculum (fish)|operculum]] (gill cover). Although countercurrent exchange systems theoretically allow an almost complete transfer of a respiratory gas from one side of the exchanger to the other, in fish less than 80% of the oxygen in the water flowing over the gills is generally transferred to the blood.<ref name=campbell />