Fish have gills because water holds far less oxygen than air, and extracting that small amount requires a specialized organ with enormous surface area and a clever flow design. Air is about 21% oxygen. Water, by comparison, contains roughly ten oxygen molecules per million water molecules, topping out around 8 milligrams per liter in typical surface waters during summer. Gills solve this problem by passing water over thin, blood-rich tissue in a way that pulls oxygen out with remarkable efficiency.
How Gills Extract Oxygen From Water
A fish’s gills sit behind the head, protected under bony flaps called opercula. Inside, each gill arch supports rows of gill filaments, and each filament is lined with tiny folds called lamellae. These lamellae are where the real work happens. They’re extremely thin, packed with blood vessels, and collectively create a huge surface area relative to the fish’s body size. More active fish, like tuna, have longer filaments and more tightly packed lamellae than slower species, giving them a larger total exchange surface.
The key to the gill’s efficiency is that blood flows through the lamellae in the opposite direction to the water passing over them. This arrangement, called countercurrent exchange, means blood that is nearly saturated with oxygen still encounters water that has even more oxygen to give up. At every point along the lamella, there’s a concentration difference pushing oxygen from the water into the blood. The result is striking: fish and crustaceans using countercurrent exchange can absorb up to 90% of the dissolved oxygen in the water flowing over their gills. If blood and water flowed in the same direction, the system would hit an equilibrium partway through and waste much of the available oxygen.
Gills Do More Than Breathe
Oxygen uptake gets most of the attention, but gills handle several other jobs simultaneously. Carbon dioxide leaves the blood the same way oxygen enters, diffusing across the thin lamellae into the passing water. Because fish pump such large volumes of water to get enough oxygen, they’re effectively over-ventilated for carbon dioxide removal. The gas exits easily.
Ammonia, the main waste product of protein metabolism in fish, also exits through the gills. In freshwater species, ammonia gas diffuses directly from the blood across the gill tissue and into the surrounding water. The process gets a boost from an elegant chemical trick: as carbon dioxide leaves the gills and reacts with water on the outer surface, it generates hydrogen ions that trap ammonia in a form that can’t diffuse back in. This keeps the concentration gradient favorable and waste flowing outward. Saltwater fish use a similar diffusion pathway but also rely on “leakier” routes between cells, since the chemistry of salt water changes the equation slightly.
Gills also regulate salt and water balance. Specialized cells called ionocytes actively transport sodium, chloride, and calcium ions across the gill surface. A freshwater fish constantly loses salts to its dilute surroundings, so its ionocytes pump ions inward. A saltwater fish faces the opposite problem, absorbing excess salt that must be pumped back out. The gill adjusts its transport machinery depending on the environment, which is one reason some species can move between fresh and salt water.
How Fish Push Water Over Their Gills
Most fish use a two-pump system to keep water flowing. A buccal pump in the mouth pushes water backward, while an opercular pump behind the gills pulls it through. These two pumps work slightly out of sync so that water moves across the gills almost continuously, even between breaths. It’s a bit like breathing in and out at the same time.
Some fast-swimming species take a different approach. Tuna, certain sharks, and other constantly moving fish simply hold their mouths open and let forward motion force water over their gills. This is called ram ventilation, and for a small number of species it’s the only option. Obligate ram ventilators must keep swimming to breathe. If they stop, water stops flowing and oxygen delivery drops.
Why Gills Don’t Work in Air
Gills are beautifully adapted for water, but that same design fails on land. The lamellae are so thin and delicate that without the buoyant support of water, they collapse against each other. Surface tension pulls the tissue together, drastically reducing the exposed area available for gas exchange. A fish out of water isn’t just missing its oxygen source. Its breathing organ is physically shutting down.
Some fish that spend time on land or in low-oxygen water have evolved partial workarounds. Certain species have stiffer gills with small projections that act as spacers between lamellae, preventing collapse in air and reducing water loss through evaporation. Lungfish and a few other groups have external gills during larval stages, or even functional lungs alongside their gills. But for the vast majority of fish, the gill is an all-in commitment to underwater life.
Why Active Fish Have Bigger Gills
Gill size tracks closely with lifestyle. A fast, open-water predator like a mackerel needs far more oxygen per minute than a bottom-dwelling flatfish, and its gills reflect that. Active species have longer total filament length, more numerous lamellae packed closely together, and thinner tissue barriers between blood and water. These features increase both the total surface area and the rate of diffusion at each point along it.
Sluggish species go the other direction. Their lamellae are more widely spaced and taller individually, but there are fewer of them and the total filament length is shorter. This design trades peak oxygen capacity for lower resistance to water flow, reducing the energy the fish spends on pumping water. It’s a trade-off tuned to demand: a fish that rests on the seafloor simply doesn’t need the same oxygen throughput as one chasing prey at full speed.
Where Gills Came From
Gills didn’t appear out of nowhere. They evolved from structures called pharyngeal arches, a series of paired tissue segments that form on the sides of the embryonic head in all vertebrate embryos, including humans. In fish, some of these arches develop into gill-supporting skeletons. In land vertebrates, the same embryonic structures become parts of the jaw, the tiny bones of the middle ear, and the cartilage of the larynx.
Fossil evidence suggests that ancient jawless vertebrates had gill arches extending much farther down the body than in modern fish. Over time, these structures became more specialized and concentrated near the head. The same developmental toolkit that builds gills also contributes to paired fins, which is why researchers consider gill arches and fins to be “serial homologs,” structures that share a deep developmental origin even though they look and function very differently in adult fish.