Home Fish Science Why Can’t Fish Breathe out of Water? (Quick Read)

Why Can’t Fish Breathe out of Water? (Quick Read)

by gvald

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


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


What is the difference between human breathing and fish breathing?

Breathing simply means catching oxygen (found in water or in the air) and sending it to our bloodstream so that all our cells can have it, otherwise they die. 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 mouth and nose. This air then reaches the lungs, where the oxygen, one of the gases present in the air, passes into our blood (to be more precise, the oxygen passes into the blood at the level of the pulmonary alveoli).


The oxygen is then taken in charge by very particular blood cells: the red blood cells. These cells allow the transport of oxygen to all the cells of our body.


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


How do fish breathe underwater?

Fish swallow water through their mouths and then let it out through their gills, which are made of several sheets that can be seen under the fish’s gills. The gills are very red because they contain many small blood vessels to ensure a maximum of exchanges between the air and the blood.


The sheets are made of a very large number of lamellae to optimize the exchange surface. The water will then circulate in the gills which will capture oxygen and send back carbon dioxide (a bit like us when we breathe, we inhale air to capture oxygen and then we blow back carbon dioxide).


Fish have to make an even greater effort than we do because water is much denser and more viscous than air. Moreover, there is less oxygen in water than in air. So they have to swallow almost 500 liters of water (!) to get 1 liter of oxygen.


For information, we need to suck in 20 times less to get the same amount of oxygen… Some fish also breathe partly through their skin, protected by a slimy layer called mucus.


How does a gill work?

Concretely, how does it work? The fish breathes through its mouth, that’s probably why it opens it so much. When it breathes in, the fish brings water into its mouth. It then closes its mouth and the water passes into the gills, which at this moment recovers oxygen. Then, the next moment, the water comes out through the gills.


That’s why, unlike us, fish can’t drown: water circulates quickly. As soon as it enters, it leaves again! While our lungs are capable of capturing the oxygen present in the water, they cannot evacuate the water. This is the only reason why it is impossible for us to breathe underwater.


Surprisingly, a small proportion of human beings are still born with a congenital anomaly called the preauricular sinus, which is characterized by a hole in the anti-helix of the ear. This could be an evolutionary remnant of gills from our ancestors.


Can fish breathe out of water?

No, fish cannot breathe out of water, however, this is not only the case for marine mammals, which have lungs. Dolphins and whales regularly come up to the surface to get some air, just like us at the pool. Except that we will have difficulty staying 50 minutes in apnea like the whale.


How do fish breathe through their skin?

Many fish, like trout or goldfish, absorb oxygen through their skin: this is called skin breathing. They can absorb up to one third of the oxygen they need in this way.


The skin of fish is covered with a slimy substance called mucus. The mucus has a protective role: it prevents micro-organisms from penetrating the fish’s skin and causing diseases. In order not to damage this protective layer and not to interfere with breathing, you should avoid touching the fish as much as possible.


Can fish live in sparkling water?

To know if the fish can breathe in sparkling water, it is necessary to know the nature of the gas contained in it. Because sparkling water is not filled with air (oxygen) but with… carbon dioxide (CO2), the gas released by humans and fish when they breathe.


It can become dangerous or even deadly if its concentration is too high, which is the case in carbonated water. The fish will succumb because it will not be able to breathe and will be intoxicated by the CO2.

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