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Fish breathe by moving water over their gills. Dissolved oxygen passes from the water into the blood, while carbon dioxide passes from the blood into the water. They do not get oxygen by splitting water molecules.
How fish breathe, step by step
- Water reaches the gills. Many bony fish draw water in through the mouth and move it through the gill chamber using coordinated mouth and gill-cover movements. The gill cover is called an operculum. The Iowa Department of Natural Resources and PetMD describe this process.
- Water flows over a broad, delicate surface. Gill arches support filaments, which bear thin structures called lamellae. Blood flows close to the water within the lamellae, giving gases a short distance to cross. A review in Physiological Reviews describes the gill’s exchange structure and other functions.
- Gases diffuse across the gills. Dissolved oxygen moves from water into blood, and carbon dioxide moves from blood into water. The blood then carries oxygen around the fish’s body. The American Museum of Natural History and Iowa DNR explain this exchange.
- Opposing flows help oxygen keep moving into the blood. In the commonly described countercurrent arrangement, blood and water flow in opposite directions along the lamellae. That arrangement maintains a difference in oxygen concentration across the exchange surface, supporting diffusion along it. AMNH and the Physiological Reviews article describe countercurrent exchange.
Where the oxygen comes from
Water contains oxygen in two different senses: oxygen is part of each water molecule, and some oxygen gas is dissolved in the water. Fish use the dissolved oxygen gas. Their gills do not pull oxygen atoms out of water molecules or break water apart. The American Museum of Natural History’s educator resource explains the distinction.
Do all fish have to swim to breathe?
No. Many fish ventilate their gills by actively moving water through the mouth and gill chamber. Some rely more on forward movement to push water over the gills, and fish have varied adaptations for ventilation. That variation does not mean every fish must swim continuously to breathe. The PetMD overview and National Park Service discuss differences in how fish move water over their gills.
How gills differ from lungs
Both gills and lungs exchange gases between an animal and its surroundings. The medium differs: fish gills take up oxygen dissolved in water, while lungs take up oxygen from air. Fish move water across the gills; lungs move air across their exchange surfaces. At fish gills, countercurrent flow can help maintain oxygen diffusion along the lamellae. Comparisons of overall efficiency depend on the species, conditions, and the measure being compared, so there is no single universal ranking.
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The Iowa Department of Natural Resources puts the basic contrast simply: “The difference is, we get our oxygen through air while fish get it through water.”
Gills do more than exchange gases
Gas exchange is the central job relevant to breathing, but gills also help regulate salt and water balance, maintain acid-base balance, and excrete nitrogenous waste. These functions make gills important to the fish’s wider internal balance, not just its oxygen supply, as described in the Physiological Reviews article.
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How much oxygen do gills extract?
The American Museum of Natural History gives a general figure of about 75 percent of the oxygen passing through the gills extracted. The page does not state a year, fish species, water conditions, or measurement method for that figure, so it should not be treated as a value that applies identically to every fish.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What this means for aquarium owners
Normal gill respiration depends on suitable water conditions. PetMD includes proper aeration among healthy aquarium conditions, but that does not mean every aquarium needs an air pump: the appropriate setup depends on the aquarium and its equipment. Filtration, aeration, and stable water chemistry are practical parts of supporting fish respiration.
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