Categories: Facts

How do fish breathe under water?

Today we answer a question from our readers: “How do fish breathe under water?

This is a good question and we thank the author!

Pixabay photo

In fact, it is quite surprising to see that fish can breathe in the water while we, are unable to stay underwater for more than a few seconds or even a few minutes to …

If you look closely, to tell you the truth, the fish are not very comfortable when you take them out of the water either… It’s all about equipment! The fish are equipped for the water and we are equipped for the air!

Breathing simply means taking in the oxygen (found in water or air) and sending it into our bloodstream so that all our cells can get it, otherwise they die…

Our bones, our muscles, our skin, our heart, our brain, in short, our whole body needs it and this constantly!

Humans breathe air through their mouths and noses. The air then reaches the lungs, where the oxygen, one of the gases in the air, passes into our bloodstream (to be more precise, the oxygen passes into the bloodstream in the alveoli of the lungs).

Oxygen is absorbed by very special blood cells: red blood cells. They allow the transport of oxygen to all the cells that make up our body.

Fish do not breathe out of water. Therefore, they must use the oxygen present in the water.

Their respiratory system is very different from ours. They do not have lungs but gills. These organs are capable of filtering the oxygen dissolved in the water and passing it into their bloodstream.

How does that work? Fish swallow water through their mouths and then pull it out through their gills, which are made up of several leaves that can be seen under the fish’s lids.

Next time you go to the fish shop or fishing, separate the lids and look what’s underneath.

The gills are very red because they contain many small blood vessels to ensure maximum exchange between air and blood.

The sheets are made of a large number of plates to optimize the exchange surface.

The water will circulate through the gills which will capture the oxygen and return the carbon dioxide (a little like us when we breathe, we breathe in air to capture the oxygen and then we blow out returning the carbon dioxide).

The air we breathe has about 21% oxygen. So we need to breathe a lot of air to have enough oxygen to live.

The fish have to make an even greater effort than we do because water is much denser and more viscous than air.

In addition, there is less oxygen in the water than in the air. They have to swallow almost 500 liters of water (!) to get 1 liter of oxygen.

For your information, we need to suck 20 times less to get the same amount of oxygen…

Some fish also breathe partly through their skin, protected by a sticky layer called mucus.

Observations and anecdotes :

Contrary to what one might think, fish do not bubble when they breathe. When you see some fish making bubbles, they are actually building nests to lay their eggs (this is the case with fighting fish, for example).

The movements they make with their mouths are used to suck in water. If the fish in your aquarium seem to breathe quickly, it is because they are short of air (dirty water, insufficient water mixture, etc.).

Beware, whales, dolphins, seals, sea lions, etc. are not fish! They are mammals that breathe like us: they have lungs and cannot breathe under water.

They are forced to surface regularly to breathe air. However, they can remain a long time between two ascents.

The whale can stay underwater for almost an hour without taking in any air!

Watch out! When babies are in their mother’s womb, they don’t breathe like fish, not even as children or adults.

It is their mother who breathes for them and sends them oxygen through the blood that passes through the umbilical cord.

However, they begin to breathe with their lungs from birth.

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