It is only natural for anyone, during the course of their time on this planet, to wonder how fish breathe. After all, humans and most other mammals need to carry oxygen with them into water, by holding their breath or by tank, just to survive a dip underneath. While fish have a different toolbox for breathing, they do share some structural similarities with other living creatures, including humans, that enable them to just keep living.
How fish breathe in water
In order for a fish to live and breathe, they require oxygen, just like us. Fish and human alike, it’s our blood that needs the oxygen, and it’s the blood that carries it all over the body to keep things running. The heart and brain are of particular concern, as oxygen deprivation to these organs is damaging at best, fatal at worst. So how do we get that oxygen delivered to the blood?
Let us have a look at lungs versus gills. Lungs are how humans breathe in oxygen, gills are how fish breathe oxygen. We have the same basic requirements, only with different respiratory systems. You breathe air through your nose and mouth, and the lungs process it through tiny blood vessels, delivering the oxygen to the blood, which in turn oxygenates every cell in your body. Your lungs then exhale all of the ‘waste’ that your body doesn’t need to go on.
So if fish need oxygen to breathe just like us, why isn’t the air good enough? The answer is because of their gills. Gills are slits present on either side of the esophagus, behind the fish’s mouth. Water, as you know from science class, is comprised of two different hydrogen atoms, along with one oxygen. Therefore, fish get all the oxygen they need to breathe from the water they’re swimming in. Of course, water is not as oxygen-rich as the air we breathe in so effortlessly, but fish are evolved to deal with this. Gills are on the surface of the fish, so they don’t have to perform any actions other than swimming to keep them working on getting the oxygen from the water, and the faster they swim, the faster the water will flow through. Therefore, swimming is the fish equivalent to taking in a big breath of air. Gills, when compared to the overall size of the fish, also have quite an impressive surface area, so a good amount of water is being processed for oxygen at all times. Fish typically sport a few gills on either side of their heads. Additionally, take into account that fish are cold-blooded creatures, whereas we humans are warm-blooded. Cold-blooded animals do not require the amounts of oxygen that we do. It is a perfect storm of favorable features that have kept fish breathing for hundreds of millions of years.
While it is our blood vessels, or capillaries, inside of the lungs that exchange gases, filaments covered in epithelial cells of the fish’s gill does the work for them, taking in oxygen and exchanging carbon dioxide. So the function is quite the same, it is the scenery that is so drastically different. And the salt content of the water is taken into account by the nature of the fish, too. Gills contain chloride cells, which disallows too much sodium from salty seawater from entering into the fish’s bloodstream. A fish that doesn’t breathe in the sea, but a river on land, has fewer of these cells, and can retain enough water to stabilize their sodium levels. Again, this is akin to a human requirement- we need salt, but too much can be fatal.
The waters flow in a way that helps fish breathe. This process is called countercurrent flow, and it’s a stroke of nature’s genius that humans have observed and emulated in industry and enterprise. It involves the blood of the fish and the water it is swimming in flowing in opposite directions. This is so effective because a fish’s heart only pumps blood in one direction. When water is rushing over the gills opposite the direction of blood flow, the water will always contain more oxygen, keeping the oxygen diffusion process going, as opposed to the fifty percent oxygen threshold satisfied in same-direction waters.
So, why don’t the gills breathe air? They could, except that they are not structurally sound for processing air, and in fact they will cave in, dry up, and stick shut when exposed to air for too long. Those filaments mentioned earlier are very feathery, and can also be compared to tissue paper- either way, they are extremely delicate, and need water running over them to remain expressed. Fish die soon after being removed from the water because their gills simply cannot stay adequately moisturized and open, so they essentially, for lack of a better term, drown on dry land.
Realizing how fish breathe, how the water contains everything they need, begs the question of water quality. Fish either cannot breathe in heavily polluted waters, or are poisoned by it, and as a result they die. Suffocation happens when the pollution in the water is at such a level that the oxygen is depleted. This too is like drowning, and is a horrible way to die. About seventy percent of the planet is covered in water, and around seventy percent of industrial waste is dumped into those very waters. In the United States alone, nearly half of the rivers are considered too polluted for aquatic life to thrive. Fish species are going extinct at a rate five times that of land animals, and with climate change and lax environmental regulations on corporations that produce chemicals, as well as nuclear waste, those numbers will only continue to climb. Humans can help by recycling, composting, and never littering, but calling attention to the waste disposal practices of industries in your area is a very effective way of making your community think about the future of the fish they eat. A fish can only breathe if we let them.