How do fish breathe?

From the May/June 2017 issue of Iowa Outdoors magazine
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But Why? 
Helping adults answer children’s nature questions

Jimmy, 12, in Keosauqua, asks:
How do fish breathe?

Humans and fish both require oxygen to survive. The difference is, we get our oxygen through air while fish get it through water.

With humans, the lungs extract oxygen from the air we breathe and transfer it into the bloodstream, at the same time releasing carbon dioxide from the system.

With most—but not all—fish, the gills do the same thing. Fish take water into their mouth, passing the gills just behind its head on each side. Dissolved oxygen is absorbed from—and carbon dioxide released to—the water, which is then dispelled.

The gills are fairly large, with thousands of small blood vessels, which maximizes the amount of oxygen extracted. “Breathing” is much harder for fish, though, because the amount of oxygen in the air versus the amount in water is substantially higher.