How do fish breathe under water?

Cú Bông tried holding its breath like a fish, but soon had to breathe out. The goldfish in the pond, meanwhile, swam about under water all day.

Cú Bông guessed that fish breathe the bubbles rising through the water. But the village pond had no bubbler, and the fish swam about quite happily.

Looking closer, Cú Bông saw the fish's mouth opening and closing. Two small covers on either side of its head opened and closed as well.

Water has oxygen dissolved in it, the way sugar dissolves in a drink. People and fish both need oxygen to live; fish take it in through their gills.

In a goldfish, water goes in at the mouth, over the gills and out under the gill covers. Gills are made of very thin layers laced with tiny blood vessels. The oxygen dissolved in the water passes through those layers into the fish's blood.

Press See the water flow to follow the water across the gills. Watch where the oxygen leaves the water and passes through the thin gill tissue.

You might not know

People take their oxygen from the air. Fish take the oxygen dissolved in water, through their gills.
Some fish must keep swimming to push water across their gills. Others can pump water across them even while keeping still.

Fish breathe with their gills. As water flows across them, the oxygen dissolved in it passes into the fish's blood.

The chapter's sources

Everything this chapter tells you rests on at least two trustworthy sources, retold in the book's own words.

Fun Facts About Fascinating Fish — NOAA Fisheries
How do gills work? — The Conversation, Curious Kids
Blue in the Face: Breathing Underwater — U.S. National Park Service
Do fish sleep? Here's what scientists have discovered so far — National Geographic