How do fish breathe?

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Fish spend their whole lives underwater and yet they need oxygen just like us. How do they breathe without lungs? The answer lies in very special organs: gills.

Oxygen is in the water too

The water of rivers, lakes and seas carries dissolved oxygen, although much less than air. Fish have developed a very effective way of using that oxygen.

What gills are

Gills are on the sides of the head, under flaps called opercula. They look like red combs, because they are full of very fine filaments and tiny blood vessels. The more surface they have, the more oxygen they can absorb.

How it works

The fish opens its mouth and takes in water. The water flows over the gills, where oxygen passes through their thin skin into the blood, while carbon dioxide goes out into the water. Then the water leaves at the sides, under the opercula. That is why you see fish opening and closing their mouths nonstop.

Why they cannot breathe out of water

Out of water, the gills stick together and dry out, and they can no longer absorb oxygen from the air. That is why a fish out of the water suffocates, even though there is plenty of air around it.

Fish that breathe air

Some fish have extra tricks. Lungfish have lungs and can spend time out of the water, and fighting fish, or bettas, rise to the surface to gulp air with a special organ.

Sharks that never stop

Some sharks must keep swimming so that water flows into their mouth and over their gills. Others can pump water and rest on the sea floor.

A fun fact

Fish also need the water to contain enough oxygen. In aquariums, filters and air bubblers are used so the water is renewed and the fish can breathe well.