What Is a Gill and How Does It Work?

A gill is a respiratory organ that extracts dissolved oxygen from water and releases carbon dioxide back into it, functioning as the aquatic equivalent of a lung. The basic working unit is a thin, plate-like structure called a lamella, densely packed with tiny blood vessels and covered by an extremely thin layer of tissue so that gases can pass through with minimal resistance. But gills do far more than breathe. They regulate salt and water balance, dump metabolic waste like ammonia, and help control blood pH, making them one of the most multitasking organs in the animal kingdom.

The Anatomy of a Fish Gill

If you lift a fish’s gill cover (the operculum), you see a set of curved, bony supports called gill arches. Most bony fish have four pairs, one set on each side of the head. Each arch carries two rows of long, fleshy projections called gill filaments, which fan out like the teeth of a comb. And on each filament sit dozens of even smaller plates called lamellae, stacked like pages in a book. This layered design creates an enormous surface area packed into a small space.

The shapes of gill arches vary across species and feeding styles. Some are hook-shaped, others semilunar or crescent-shaped, and these differences relate partly to how a fish captures food, since the arches also anchor gill rakers, the comb-like projections that filter particles from water before they reach the delicate lamellae.¹ The filament is the core functional unit, and the lamellae on its surface are where the real work happens. Each lamella is built around a network of pillar cell capillaries, giving blood and water an extremely short distance to bridge for gas exchange.²

How Gas Exchange Actually Works

The elegant trick behind gill efficiency is something called countercurrent flow. Water moves over the lamellae in one direction while blood flows through them in the opposite direction. This matters because it keeps a difference in oxygen concentration along the entire length of the exchange surface. If water and blood flowed the same way, oxygen would transfer only until the two reached equal levels, leaving a lot of dissolved oxygen unused. By running the streams in opposite directions, even blood that has already picked up some oxygen keeps encountering water that still has more to give.

This mechanism is ancient. Computational modeling of a Late Ordovician trilobite, an arthropod that lived roughly 450 million years ago, suggests that countercurrent flow was already at work in its gills. During certain gill strokes, water flowed opposite to the direction of the animal’s blood-like fluid, creating the conditions for highly efficient oxygen uptake.³ The same basic principle shows up in modern fish, in bird lungs, and even in some engineered heat exchangers. It is one of evolution’s most repeated good ideas.

Salt Balance and Why Gills Are Not Just for Breathing

A freshwater fish faces a constant flood of water entering its body through osmosis, while its internal salts try to leak out. A saltwater fish has the opposite problem: water leaves its body, and salts pour in. In both cases, gills are the frontline defense. Specialized cells in the gill epithelium, called ionocytes, actively pump ions in or out depending on the environment. These cells use distinct sets of molecular transporters to absorb sodium, chloride, and calcium in freshwater, or to secrete excess sodium chloride in seawater.⁴

Research in zebrafish has identified at least five types of ionocytes in the gills, each dedicated to a different ion-transport job. One type handles sodium uptake and acid secretion. Another manages calcium. A third takes care of potassium secretion. This division of labor means the gill can fine-tune each ion independently, responding to changes in water chemistry without disrupting the others.⁵

Species that migrate between fresh and salt water, like salmon, need to flip this entire system. Studies using antibodies specific to freshwater and seawater versions of a key enzyme have shown that salmon gills can distinguish between ion-absorbing and ion-secreting ionocytes, essentially retooling themselves as the fish moves from river to ocean and back.⁶ That kind of flexibility is remarkable for a single organ.

Waste Removal Through the Gills

Most fish produce ammonia as their primary nitrogen waste, a byproduct of breaking down proteins. Rather than converting it into less toxic compounds the way mammals do with urea, fish simply dump ammonia straight into the water, and the gills are the main exit route.⁷ This works because fish are continuously flushing large volumes of water over their gills, diluting the ammonia as fast as it leaves.

The molecular machinery behind this turns out to be surprisingly complex. Specialized transport proteins called Rhesus glycoproteins sit in the gill epithelium and shuttle ammonia molecules across the cell membranes and into the water.⁸ In the climbing perch, a fish that can survive out of water for extended periods, researchers found that a sodium-hydrogen exchanger in certain gill ionocytes works alongside these ammonia-transporting proteins to facilitate proton-assisted ammonia excretion, linking waste removal to acid-base balance.⁹ Ammonia excretion, ion regulation, and pH control are not three separate jobs at the gills so much as three outputs of one interlocking system.

Gills That Reshape Themselves

Some fish can physically remodel their gills in response to how much oxygen is available. Goldfish exposed to severely low-oxygen water for about a week shed a mass of cells that normally fills the spaces between their lamellae, exposing more surface area to the water.¹⁰ The same pattern has been documented in largemouth bass, where low oxygen triggers a significant increase in respiratory surface area. When oxygen levels return to normal, the remodeling reverses within about a week, and the interlamellar tissue grows back.¹¹

Why keep the extra cell mass around at all? The interlamellar tissue likely protects the delicate lamellae from parasites, pathogens, and waterborne irritants. It also reduces the surface area across which ions can leak, helping with osmotic balance. So there is a genuine trade-off: maximizing breathing capacity versus minimizing exposure to the outside world. The fish reshuffles this balance depending on what threatens it most at any given time. Blunt snout bream show the same reversible response, with the interlamellar cell mass shrinking significantly under low oxygen and fully recovering after a week of normal conditions.¹²

Gill Size and a Fish’s Metabolic Ceiling

A popular idea in fish biology, sometimes called the gill-oxygen limitation hypothesis, proposes that as a fish grows larger, its gill surface area cannot keep up with its rising oxygen demand. The prediction is that gills eventually become a bottleneck, capping how big or how active a fish can be. It is an appealing concept, especially in discussions about how warming oceans might shrink fish body sizes by increasing metabolic demand while gill capacity stays flat.

The evidence, though, is more complicated. An analysis of how gill surface area scales relative to metabolic rate across multiple species found that the ratio of gill area to oxygen demand stays roughly constant as fish grow, rather than declining the way the hypothesis predicts. In other words, gills appear to keep pace with the body’s needs throughout life for most species studied.¹³ That said, the same research showed that differences in gill area between species can explain about a quarter of the variation in their metabolic rates, which means gill size still matters at the species level even if it is not the universal growth constraint within a species that some researchers have proposed.

How Ancient Are Gills

Gills are among the oldest innovations in animal evolution. Fossils of vetulicolians, strange marine creatures from the Cambrian period over 500 million years ago, preserve what appear to be pharyngeal gill slits, openings in the throat region that allowed water to flow through for respiration and possibly feeding. The evolution of these openings is considered one of the key innovations behind the success of deuterostomes, the enormous lineage that includes everything from sea stars to humans.¹⁴

The earliest gills likely served a dual purpose: filtering food particles from seawater and absorbing some oxygen as a bonus. Structural support for these pharyngeal gills appears in nearly all deuterostomes that have them, originally in the form of a collagen-based acellular skeleton. The ancestral deuterostome was probably a worm-like creature whose pharyngeal openings were braced by this collagenous framework.¹⁵ Over hundreds of millions of years, that simple scaffolding evolved into the bony or cartilaginous gill arches of modern fish.

When Gills Meet Land

Not every gill-bearing animal stays fully submerged. Mudskippers, climbing perch, and other amphibious fish spend significant time out of water, and their gills present a design problem: the thin lamellae that work beautifully in water tend to collapse and stick together in air, drastically reducing surface area. These species have evolved workarounds. Some use their skin for a large share of gas exchange on land. Others have modified gill chambers or specialized mouth-lining tissue with increased blood vessel density that enhances oxygen uptake during air exposure.

Amphibian larvae take a different approach entirely. Tadpoles of frogs and toads breathe through external or internal gills that are gradually replaced by lungs as the animal metamorphoses. The transition from gill to lung breathing during metamorphosis is one of the most vivid examples of organ-system overhaul in the vertebrate world, and it roughly mirrors what happened over evolutionary time as vertebrates moved from water to land. Human embryos briefly develop pharyngeal arches that are remnants of this ancestral gill apparatus, though they never function as gills. Instead they give rise to structures of the jaw, ear, and throat.

Environmental Threats to Gill Health

Because gills are in continuous, intimate contact with the surrounding water, they are among the first tissues to suffer when water quality deteriorates. Heavy metals, pesticides, and microplastics all concentrate at the gill surface. In zebrafish exposed to microplastics alongside cadmium, a toxic heavy metal, the gills accumulated significantly more cadmium than they did when exposed to the metal alone, and the combination caused oxidative damage and inflammation in gill tissue.¹⁶ Long-term exposure to microplastics with copper triggered changes in gene expression related to cell death pathways in zebrafish gills, suggesting chronic stress at the molecular level.¹⁷

Disease poses its own risks. Amoebic gill disease, caused by a single-celled parasite, is a serious problem in farmed Atlantic salmon. Infected fish show measurably lower blood oxygen levels and higher carbon dioxide levels compared to healthy fish, even under normal oxygen conditions. The parasite essentially impairs the gill’s gas-transfer ability. Interestingly, during experimental low-oxygen challenges, the infected fish did not suffer outright respiratory failure any faster than healthy ones, suggesting they have some compensatory capacity, but they are working from a deficit at all times.¹⁸

Artificial Gills and Engineering Inspired by Biology

The efficiency of biological gills has inspired engineers to ask whether humans could extract dissolved oxygen from water without surfacing. Several research groups have explored so-called artificial gill systems that mimic the plastron, the thin air layer trapped by hairs on the bodies of certain aquatic insects. The idea is to create a membrane that lets dissolved oxygen pass through into an air space while keeping water out, effectively letting a diver breathe from the water itself.¹⁹

The challenge is scale. A resting human needs roughly 15 to 20 times more oxygen per minute than even a large, active fish, and dissolved oxygen in water is far less concentrated than in air. To extract enough oxygen for a person, an artificial gill would need an impractically huge membrane surface or an enormous volume of water flowing past it every second. Current prototypes work at proof-of-concept scale but are nowhere near supporting human respiration. The concept remains more promising for small-scale applications, like oxygenating enclosed aquatic habitats or extending the operational time of underwater sensors, than for replacing scuba tanks.

Gills Beyond Fish

Fish gills get most of the attention, but the gill concept appears across a huge range of aquatic life. Crustaceans like crabs and lobsters breathe through gills housed inside a chamber beneath their carapace, pumping water across them with specialized appendages. Many aquatic insect larvae have tracheal gills, feathery outgrowths along their abdomens that absorb oxygen directly into their respiratory tubes. Bivalves like mussels and clams use large, flat gills not only for breathing but simultaneously for filtering food particles from the water, making the gill do double duty as both lung and mouth.

Nudibranchs, the vividly colored sea slugs, often carry their gills as exposed plumes on their backs, a strategy that maximizes gas exchange but leaves the gills vulnerable to predators. Some species of sea cucumber breathe through respiratory trees, branching structures inside the body that draw water in through the anus, a creative if undignified solution to the same gas-exchange problem that gills solve externally. The diversity of gill designs across the animal kingdom reflects half a billion years of independent tinkering with the same fundamental physics: thin membranes, large surfaces, close contact between blood and oxygenated water, and wherever possible, countercurrent flow to squeeze the most oxygen out of every pass.