How do fish breathe underwater?

Indeed it is quite astonishing to see that the fish can breathe in the water whereas we are unable to stay the head underwater more than a few seconds or even a few minutes for the strongest …

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

Breathing it simply means catching the dioxygen (found in water or in the air) and sending it into our blood so that all our cells can have it, otherwise they dieā€¦.Our bones, our muscles, our skin, our heart, our brain in short, our whole body needs it constantly!

We humans breathe in air through our mouths and noses. The latter will then reach the lungs, where the dioxygen, one of the gases present in the air, passes into our blood (to be more precise, the dioxygen passes into the blood at the level of the pulmonary alveoli).

The dioxygen is then taken care of by very specific blood cells: red blood cells. These allow the transport of dioxygen to all the cells constituting our body. We had already spoken a little about it HERE .

  • Fish do not breathe outside the water.
  • They must therefore use the oxygen present in the water!
  • Their respiratory system is therefore very different from ours.
  • They don’t have lungs but gills.
  • These organs are able to filter oxygen dissolved in water in order to pass it into their blood.

How it works ? Fish swallow water through the mouth and then bring it out through the gills, made up of several layers that can be seen under the gill covers of the fish. The next time you go to the fishmonger or fishing, pull the lids aside and look at what is below (as in the photo).


The sheets themselves are made from a very large number of strips to optimize the exchange surface. The water will therefore circulate in the gills which will capture the oxygen and return carbon dioxide (a bit like us when we breathe, we suck air to capture oxygen and then we breathe by returning carbon dioxide ).

fish2In the air we breathe there is about 21% oxygen. It is therefore necessary to inhale a lot of air to have a sufficient quantity of oxygen to live.

The fish have to exert an even greater effort than us because the water is much more dense and viscous than the air. In addition, there is less oxygen in the water than in the air. They therefore have to swallow almost 500 liters of water (!) To obtain 1 liter of oxygen. FYI, we need to suck 20 times less to get the same amount of oxygen … Some fish also breathe partly through the skin, protected by a slightly sticky layer called mucus.

Notes and anecdotes :

Contrary to what you might think, fish do not bubble when they breathe. When you see some fish making bubbles, they actually build nests to lay their eggs (like fighting fish, for example). The movements they make with their mouths are used to suck up water. If the fish in your aquarium seem to breathe quickly, it is because they lack air (dirty water, not enough water mixing, etc.).

Beware, whales, dolphins, seals, sea lions etc. are not fish! They are mammals that breathe like us: they have lungs and cannot breathe underwater. They are forced to come to the surface regularly to breathe in air. However, they can stay for a very long time between two lifts. The whale can stay underwater for almost an hour without regaining air!

Warning ! when babies are in their mother’s womb, they don’t breathe like fish, or even like children or adults for that matter. It is their mother who breathes for them and sends them oxygen through the blood that passes through the umbilical cord. On the other hand, they start to breathe with their lungs from birth.

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