Fish gills are multifunctional organs. They take dissolved oxygen from water and release carbon dioxide, but they also remove much of the fish’s nitrogenous waste, regulate salts and water, and help maintain the blood’s acid–base balance. In most fish, gills are the main exchange surface between the animal and its environment.
Water flows across thin, capillary-rich structures called gill lamellae. Oxygen diffuses into the blood, while carbon dioxide and other substances move out. The same large, delicate surface that makes gills efficient also makes them vulnerable to poor water quality, parasites, suspended particles, and physical damage.
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How fish gills take in oxygen
Fish do not obtain oxygen by splitting water molecules. They absorb dissolved molecular oxygen—oxygen gas that is present in the water—from the water passing over their gills.
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In most bony fish, water enters through the mouth and is pushed across the gills as the mouth and the operculum, or gill cover, move in a coordinated rhythm. The water then exits beneath or behind the operculum. This continuous flow brings fresh, oxygen-containing water to the gill surface.
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Inside the gill, the main supporting structures are:
- Gill arches: curved supports that hold the gill tissues.
- Gill filaments: numerous narrow projections arranged along each arch.
- Secondary lamellae: tiny folds on the filaments where most gas exchange occurs.
- Capillaries: very small blood vessels running close to the water-facing surface of the lamellae.
The lamellae provide a very large exchange area while keeping the water and blood separated by a short, thin barrier. Oxygen therefore diffuses from the water, where its concentration is higher, into blood with a lower oxygen concentration. At the same time, carbon dioxide diffuses from the blood into the surrounding water.
This structure-and-function relationship is described in more detail by OpenStax’s explanation of gas-exchange systems.
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Many fish use countercurrent exchange. Water flows across the lamellae in one direction while blood flows through their capillaries in the opposite direction. Because the blood continually encounters water with a comparatively favorable oxygen gradient, oxygen can continue diffusing into the blood along much of the lamella.
This arrangement is more effective than moving water and blood in the same direction would be. It does not mean that every fish has identical gill anatomy or extracts oxygen with identical efficiency. Species, temperature, salinity, activity, water flow, and the amount of dissolved oxygen all affect performance.
How oxygenated blood reaches the body
Fish generally have a single circulatory circuit. The heart pumps oxygen-poor blood to the gills first. After the blood is oxygenated there, it travels to the rest of the body before returning to the heart.
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This differs from the mammalian arrangement, which has separate pulmonary and systemic circuits: one sends blood to the lungs and back to the heart, and another sends it from the heart to the body. In fish, the gills are directly incorporated into the route between the heart and the tissues.
Gills remove carbon dioxide
Cells produce carbon dioxide during metabolism. If it accumulated in the blood, it would interfere with normal physiology. Because gill lamellae are thin and continuously exposed to flowing water, carbon dioxide can diffuse from the blood into the water and leave the fish.
For most fish, the gills are the principal respiratory surface for both oxygen uptake and carbon-dioxide removal. Some species, however, supplement gill respiration with other organs, including skin, lungs, modified swim bladders, or parts of the digestive tract.
Gills excrete ammonia
Protein metabolism produces nitrogenous waste. In many fish, especially many bony fish, much of this waste is eliminated as ammonia. Ammonia is toxic at elevated concentrations, so removing it quickly is important.
The gills are well suited to this job because they have:
- a large surface area;
- high blood flow;
- rapid ventilation by moving water;
- a short distance between blood and water; and
- specialized cells and transport pathways in the gill epithelium.
A substantial amount of ammonia can cross the branchial, or gill, epithelium from the blood into the water. The details vary with species and environmental conditions. Most fish are ammonotelic, meaning they excrete most nitrogenous waste as ammonia, but this is not universal. Many sharks and rays, for example, retain or excrete more nitrogen in the form of urea. Life stage, habitat, exposure to air, salinity, pH, and other aspects of water chemistry can also change how nitrogen is handled.
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Gills regulate salts and water
Fish must constantly manage the movement of water and dissolved ions across their bodies. This process is called osmoregulation, or more specifically ionoregulation when referring to the control of ions such as sodium and chloride.
Freshwater fish
Freshwater is less concentrated in dissolved salts than a fish’s internal fluids. As a result, water tends to enter a freshwater fish, while ions tend to diffuse out. Freshwater fish generally compensate by:
- actively taking up ions through specialized gill cells;
- drinking little or no water compared with marine bony fish; and
- producing a relatively abundant, dilute urine.
Marine bony fish
Seawater is more concentrated in salts than the body fluids of most marine bony fish. These fish tend to lose water and gain salts. They commonly drink seawater, absorb water through the digestive system, and use the gills—along with the kidneys and intestine—to eliminate excess ions.
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For an overview of the relationship between aquatic animals and their osmotic environment, see OpenStax’s discussion of osmoregulation and osmotic balance.
Gills help control blood pH
Gill tissue also contributes to acid–base regulation. By helping move carbon dioxide and related chemical forms, and by exchanging ions across the gill epithelium, the gills help keep the acidity of the blood within a workable range.
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This is why it is more accurate to call gills a multifunctional homeostatic organ than simply the fish equivalent of lungs. Respiration, waste excretion, ion regulation, and acid–base balance all depend on the same general interface with the water.
Why gills are sensitive to poor water quality
Gills need a broad, thin, highly exposed surface to work efficiently. Those same properties also make them vulnerable. Pollutants, suspended sediment, parasites, excess mucus, unsuitable pH or salinity, and other unfavorable water conditions can interfere with the gill surface.
Gill damage may affect several functions at once: oxygen uptake can become more difficult, carbon-dioxide removal can be impaired, and the fish may struggle to regulate salts or eliminate ammonia. This is a physiological consequence of the gill serving as a shared exchange surface, not evidence that every water-quality problem produces the same visible symptoms.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Do all fish rely on gills in exactly the same way?
No. “Fish” includes animals with substantially different respiratory and nitrogen-excretion strategies.
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- Air-breathing fish may use lungs, modified swim bladders, skin, intestines, or other accessory organs to obtain oxygen from air. Their gills can still remain important for carbon-dioxide removal, ion regulation, acid–base balance, and nitrogenous-waste excretion.
- Sharks and rays differ from most bony fish in several aspects of nitrogen handling, including their greater use of urea.
- Larval fish may rely heavily on their skin before the gills are fully developed. As they grow, the relative contributions of skin and gills change, and gills can become increasingly important for ion regulation and ammonia excretion.
- Fish in different habitats face different oxygen levels, salinities, temperatures, and water chemistries, so their gills may be adapted to different demands.
Thus, “gills extract oxygen” is correct as a basic explanation, but it is incomplete. It is also safer to say “most fish” or “most bony fish” rather than presenting one physiology as universal.
A simple way to remember the functions of fish gills
Fish gills perform four closely connected jobs:
- Respiration: oxygen enters the blood and carbon dioxide leaves it.
- Waste excretion: ammonia and other substances can move from the blood into the water.
- Osmoregulation: specialized cells help control salts and water.
- Homeostasis: ion exchange and gas exchange contribute to stable blood pH and internal conditions.
The gills’ delicate architecture makes all of these exchanges possible. It also explains why clean, suitable water is essential for fish health.
Go deeper into fish biology
Students, homeschool families, and readers who want more than a basic explanation may find a fish biology textbook useful for studying gill anatomy alongside circulation, osmoregulation, metabolism, and evolution. A book is optional—the core functions can be understood without one—but it provides the broader context that a short explainer cannot.
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Do fish breathe water?
Fish do not use the oxygen atoms locked inside water molecules. Their gills remove dissolved molecular oxygen from the water and transfer it into the blood. Water is the medium carrying the oxygen to the gill surface.
Do fish gills remove ammonia?
Yes. In most fish, especially many bony fish, the gills are a principal route for eliminating ammonia produced by protein metabolism. The exact form and route of nitrogen excretion vary among species, life stages, and environmental conditions.
What happens to fish gills in freshwater versus saltwater?
Freshwater fish generally gain water and lose ions, so their gills actively take up ions and the fish produces dilute urine. Marine bony fish generally lose water and gain salts, so they drink seawater and use their gills, kidneys, and intestine to eliminate excess ions.
Are gills the only way fish get oxygen?
No. Most fish rely mainly on gills, but some supplement or replace part of their oxygen uptake with skin, lungs, modified swim bladders, intestines, or other accessory organs. Young fish may rely more heavily on their skin before the gills mature.
The Bottom Line
Bottom line: Fish gills are not just underwater lungs. They extract dissolved oxygen, release carbon dioxide, excrete much of the fish’s ammonia, regulate salts and water, and help stabilize blood pH. Their efficiency comes from thin, folded, well-supplied exchange surfaces—and that same exposure makes them especially sensitive to poor water quality.
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