Gills are the equivalent of a mammal’s lungs, says Jeffrey Malison, director of the aquaculture program at University of Wisconsin-Madison. “Their primary purpose is to exchange gases, take oxygen in and release carbon dioxide out of the fish.”
Both lungs and gills have a bed of very small blood vessels with thin walls that the gases can easily travel across. But gills have a much harder job than lungs, Malison says. “It’s a big challenge for a fish. The air we breath is 20 percent oxygen, or 200,000 parts per million.”
Water holds 4 to 8 parts per million of oxygen, he adds. “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 to the water, they may have problems controlling salt flow, Malison adds. A freshwater fish will constantly lose salt through its gills, while a salt-water fish may have to spend a lot of energy keeping excess salt out.
Despite these challenges, gills are much older than lungs, Malison says. Complex organisms with spinal columns arose in the sea hundreds of millions of years before they moved to land.