A: Gills are the equivalent of a mammal"s lungs, says JeffreyMalison, director of the aquaculture program at UW-bsci-ch.org. "Theirprimary purpose is to exchange gases, take oxygen in and releasecarbon dioxide out of the fish."
Both lungs and gills have a bed of very small blood vessels withthin walls that the gases can easily travel across. But gills havea much harder job than lungs, Malison says.
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"It"s a big challenge for a fish. The air we breath is 20percent oxygen, or 200,000 parts per million." But water holds justfour to eight parts per million of oxygen, he said.
"It takes an awful lot of work for the fish to exchange gases,particularly oxygen. It just takes a lot of energy."
Because fish must open a "terrific" amount of blood vessels tothe water, they may have problems controlling salt flow, Malisonadds.
A freshwater fish will constantly lose salt through its gills,while a saltwater fish may have to spend a lot of energy keepingexcess salt out.
Despite these challenges, gills are much older than lungs,Malison says. Complex organisms with spinal columns arose in thesea hundreds of millions of years before they moved to land.
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