Breathing underwater might sound like science fiction to us, but for fish, it’s just physics. Without lungs, without rising for air (at least most of them), and in an environment where oxygen is dissolved in mere traces, fish have evolved one of nature’s most elegant systems: gills. These delicate, feathered organs extract the oxygen they need from water—a medium 800 times denser than air and 30 times less oxygen-rich.
But the process isn’t just fascinating—it’s complex, energy-efficient, and finely tuned by millions of years of evolutionary pressure. Here’s how fish breathe.
Oxygen in Water: The Challenge
Before we dive into the mechanics, understand this: oxygen in water is scarce. Air contains about 21% oxygen by volume. In water? Less than 1%. That means fish have to extract as much as they can, using minimal energy, without drowning in their own effort. And they do it while moving, escaping predators, and sometimes living in nearly stagnant ponds.
Water doesn’t flow easily. It’s thick, and pushing it over respiratory surfaces takes work. Every breath a fish takes has to count.
Enter the Gills: Nature’s Underwater Lungs
Gills are the breathing organs of most fish. Located on either side of the fish’s head and protected by a bony flap called the operculum, gills are made of thin, delicate structures called gill filaments, which are themselves lined with even finer lamellae—thin sheets where gas exchange happens.
Blood flows through the lamellae in capillaries, tiny vessels where the surface area is maximized. At the same time, water flows over the gills, delivering oxygen and carrying away carbon dioxide. This meeting point—where blood meets water—is where oxygen moves in, and carbon dioxide moves out.
Counter-Current Exchange: The Secret to Efficiency
The brilliance of fish respiration lies in a mechanism called counter-current exchange.
In this system, water and blood flow in opposite directions. Why? Because it keeps the concentration gradient of oxygen high across the entire gill surface. At every point along the gill, the water passing over it always has more oxygen than the blood within it. This allows oxygen to continuously diffuse into the blood.
If the flows went the same way, the gradient would vanish halfway through, and oxygen absorption would plummet. With counter-current flow, fish can extract up to 85–90% of the available oxygen in the water. Human lungs, by comparison, absorb only about 25% of the oxygen we inhale.
This makes gills more efficient than lungs in terms of oxygen extraction—despite the watery handicap.
The Pumping Process
Fish breathe by pumping water over their gills. This isn’t passive. They actively open their mouths to draw water in, then close their mouths and push the water out through the operculum, over the gills.
Some species, like tuna and sharks, rely on a method called ram ventilation—they must swim forward continuously with their mouths open to force water over their gills. If they stop moving, they suffocate. These are the fish that never sleep the way we do.
Others, like most reef fish or goldfish, can breathe while stationary, using muscular movements to draw water over the gills.
Blood’s Role: Hemoglobin Still Rules
Just like humans, fish use hemoglobin in their red blood cells to carry oxygen. The oxygen diffuses across the gill membranes into the blood, where it binds to iron molecules in the hemoglobin. From there, it’s delivered to muscles, organs, and the brain.
Interestingly, some species, like Antarctic icefish, have evolved without hemoglobin entirely. Living in oxygen-rich, freezing waters, their clear blood flows slowly and passively carries enough dissolved oxygen to survive—barely.
Gills Are Fragile, Yet Powerful
The massive surface area of gills—necessary for oxygen transfer—makes them extremely delicate. They’re vulnerable to damage from toxins, pollutants, parasites, or even rapid changes in water salinity.
Some fish can temporarily shut down parts of their gill systems to conserve energy or avoid toxin exposure. Others, like salmon, undergo entire gill transformations when moving from freshwater to saltwater.
What About Fish That Breathe Air?
Some fish are evolutionary rebels.
Species like the lungfish have developed lungs in addition to gills, allowing them to gulp air during dry seasons. The Betta and gourami have a special organ called the labyrinth, which lets them breathe air from the surface—handy for oxygen-poor waters.
Then there’s the mudskipper, a fish that spends more time on land than in water. It breathes through its skin and the moist lining of its mouth, much like an amphibian.
Fish like these show us that respiration isn’t one-size-fits-all. In fact, breathing methods often reflect the extreme environments these creatures inhabit—from stagnant swamps to roaring ocean currents.
Can Fish Drown?
Yes—and it’s not a paradox.
Fish “drown” when they can’t get enough oxygen from the water. This happens when the water is too warm (warmer water holds less oxygen), too polluted, or stagnant. It can also happen if their gills are damaged or clogged.
Even though they live in water, fish need oxygen just as much as we do. Water without oxygen is as deadly to them as air without oxygen is to us.
In Summary
Fish breathe using a biological system that’s precise, efficient, and highly evolved. Their gills perform a delicate dance of fluid mechanics and molecular diffusion—extracting life from liquid. And beneath the surface of lakes, oceans, and rivers, millions of species do it in real time, without a sound.
They don’t have lungs, but they breathe just fine.
Because evolution—quietly and slowly—designed a miracle.
The Zuara Press 