If you’re a dedicated wildlife lover or a budding ecologist, you’ve likely questioned the respiratory capacities of your favorite sea creatures.
So do fish have lungs? This is a question that comes up surprisingly often. Generally, the answer is “no”.
Fish breathe primarily by taking in oxygen from the water through their gills and expelling carbon dioxide while they swim.
Most of us are pretty familiar with fish as aquatic creatures, which is to say that we see them mostly as a part of our everyday lives.
We’re used to seeing fish as living animals that swim and breathe through gills (though some aquatic varieties do have lungs, the ones from which we derive most of our food).
They can be found in all kinds of settings, from water to land to air. And if you’ve ever wondered about the respiratory systems of fish, you are not alone.
Why Do Fish Have No Lungs?
The common freshwater fish is known to have gills that are located in the neck region. These are thin membranes that are almost made of cartilage.
The gills make up the respiratory organs of fish and serve as a passageway for oxygen intake.
This enables a water-breathing animal like any fish to extract oxygen from the water around it and use it to survive.
This essentially means that fish can breathe through their skin and gills.
Aside from expelling gases, fish cannot take in air. This is because there are no lungs that would be capable of doing so.
In other words, they cannot use external air sources to breathe as mammals or birds can.
So, why do people pose this question?
The answer lies in the fact that many scientists believe that fish lack a respiratory system in the first place.
They have speculated that there are several reasons why fish do not have lungs.
For one, fish are more adapted to the gill structures than they are to having lungs.
Also, because fish have an internal metabolic heat generation mechanism and no ability to store oxygen, they might not need a respiratory system at all.
A good example is found in “prickly” starfish. In these fish, called Hexabranchus, their oxygen exchange system is located in the water vascular system.
They use a water pump to fill the cavity with oxygenated water, which is essential for survival.
Are Gills Lungs?
Gills are projections of the body that protrude outside a fish’s body and enable it to have contact with oxygen in the water.
For the gills to be effective, they must be wet. Gills are generally lined with capillaries, which contain red blood cells filled with hemoglobin.
The hemoglobin then releases oxygen into the capillaries and to a larger extent, the blood itself.
In turn, the blood passes the oxygen through its many cavities to carry it through the body. This is the only way to have oxygen transported away from the gills and into another part of a fish’s body.
In short, gills are not lungs. They are what enable a fish to breathe in water and survive.
Even though fish may have very similar respiratory problems to that of mammals, their gills are not the same.
Lungs are flow-through organs that allow gas exchange between air and blood.
Gills, on the other hand, are more like filter organs because they do not allow thorough exchange; however, they can be used as a place where there is potential air exchange but no actual movement.
How Do Fish Gills Work?
Fish gills allow the transfer of gases, such as oxygen and carbon dioxide, across the gill membranes.
The fish’s blood normally carries these two gases in a way that they can be released by the body’s cells.
This is called gas exchange and is how a fish transports oxygen throughout its body.
Gills are made up of hundreds of small, thin filaments that are arranged in parallel to each other. They are located on the outside of a fish’s body.
The filaments are joined together to create a surface area that is much larger than the gills themselves.
However, since they lack any muscles or other structures that would enable them to move or shift around, they are pretty much always in contact with water.
This means that water is constantly entering and exiting the gill tissue as it circulates through. When fish exhale, their gills absorb oxygen from the surrounding water and pull it into their bloodstreams.
This oxygen then travels through the body, giving its cells access to it.
Likewise, during inhalation, their bodies push the carbon dioxide they have produced back out of their gills and into the water where it is eventually released.
While this process is occurring, some fish have a unique method of gas exchange that other terrestrial animals do not.
This is known as counter-current exchange, in which blood vessels in the gills contain blood flowing in an opposite direction from that found in other parts of the body.
The blood vessels contain capillaries, which can absorb oxygen directly from the water. This is a pretty unique feature and one that was once thought to be nonexistent in fish.
How About Fish on Land?
Occasionally, we hear reports of fish capable of surviving outside water, such as the Koi fish, which you can find in China, Japan, and Korea.
These types of fish are known to be able to survive on land for up to a week before dying.
So how do they breathe while they are on land?
When a fish leaves the water, it immediately starts to use gill structures to extract oxygen. However, to stay alive and not suffocate, it needs air.
Koi are known to have developed a type of “evolutionary eye” system that allows them to detect air currents and then move towards them to absorb oxygen.
Koi will also change the angle of the gill structures during inhalation to allow more oxygen to be absorbed.
Thus, they can have this type of respiratory system while on land. The fact that Koi can survive without air shows that once they are out of the water they still have a way to extract oxygen from their surroundings.
Fish are not the same as mammals, birds, or even insects when it comes to the way they breathe.
They do have gills that enable them to absorb oxygen, but they also have a respiratory system in their tissues.
This means that fish do not have any lungs and that they’re not breathing in external air sources as one might first assume.
They breathe in water and survive by extracting oxygen from the water that surrounds them.
Keep in mind that not all fish have this defense mechanism, but those that do can survive in the water.
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Hi, I’m George – the founder of MarinePatch. I created this blog as marine wildlife has been my passion for many years and I’ve spent decades learning and dedicating myself to documenting all I can about the topic.