Fish obtain dissolved oxygen from water by moving water over their gills. Oxygen crosses the thin gill surface into the blood, while carbon dioxide moves from the blood into the water. In biology, “respiration” can also mean the process cells use to release energy, so it helps to distinguish ventilation, gas exchange, and cellular respiration.
What do fish breathe?
Most fish take up dissolved molecular oxygen—oxygen mixed into the water. They do not get oxygen by breaking apart water molecules or using the oxygen chemically bound in H2O. Water must move over the gills to replenish oxygen at the exchange surface.
Three related terms describe different steps:
- Ventilation is the movement of water over a respiratory surface.
- Gas exchange is the movement of oxygen and carbon dioxide across that surface.
- Cellular respiration is the energy-releasing process inside cells, which uses oxygen and produces carbon dioxide.
Arizona State University’s Ask A Biologist distinguishes ventilation and gas exchange from cellular respiration. The gills connect these processes by taking oxygen from water and releasing carbon dioxide back into it.
How fish gills exchange gases
Gills have filaments covered with many fine, thin folds called lamellae. These provide a large surface area, with blood vessels close to the water. As water passes over the lamellae, oxygen diffuses into the blood and carbon dioxide diffuses out of it. The American Museum of Natural History describes this exchange across the thin gill membranes.
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At the exchange surface, blood generally flows in the opposite direction from water. This is called counter-current flow. Because blood encounters water with a higher oxygen concentration along the surface, a useful diffusion gradient is maintained and gas exchange remains effective.
How fish move water over their gills
Fish use different ventilation methods. Many bony fish coordinate movements of the mouth and operculum—the bony cover over the gills—to pump water across the gills. Some fish rely more on water driven through the mouth as they swim, while other species can pump water even when stationary.
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| Fish group or method | How water reaches the gills | Notes |
|---|---|---|
| Many bony fish | Mouth and operculum movements pump water over the gills. | Some also swim with the mouth open to move water through. |
| Many sharks | Forward swimming can drive water through the gills; this is called ram ventilation. | Some sharks can also pump water while stationary. |
| Some bottom-dwelling sharks | Water may enter through spiracles as well as the mouth. | Spiracles can help with ventilation while resting or feeding. |
| Jawless fish | Water can pass through a nasopharyngeal duct to gill pouches. | Hagfish can also take up oxygen through their skin. |
The U.S. National Park Service explains that fish that rely on forward movement need to swim to push water through their gills. This does not apply to every fish: many pump water actively, and some sharks can ventilate while still.
Do all fish breathe only through gills?
No. Gills are the usual respiratory route, but some fish supplement gill exchange with other surfaces or organs. Examples include oxygen uptake through the skin, a gas bladder that functions like a lung, and other supplementary routes. The American Museum of Natural History describes tarpon using a gas bladder and mudskippers exchanging oxygen through their skin; hagfish can also take up oxygen through skin.
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These are adaptations found in particular fish, not the standard arrangement for all fish. Gills remain central to respiration in most species, and the exact contribution of supplementary surfaces varies.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Frequently Asked Questions
Do fish get oxygen from water molecules?
No. Fish take up dissolved molecular oxygen mixed into the water, not oxygen chemically bound in H2O.
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What is the difference between ventilation and gas exchange?
Ventilation moves water over a respiratory surface. Gas exchange is the diffusion of oxygen into the body and carbon dioxide out across that surface.
Do all fish have to swim to breathe?
No. Many fish pump water over their gills using movements of the mouth and operculum. Some fish, including many sharks, rely more on forward swimming, while some sharks can pump water when stationary.
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Can fish breathe through their skin?
Some fish can take up oxygen through the skin as a supplementary route. The American Museum of Natural History gives mudskippers as an example, and the National Park Service notes skin uptake in hagfish.
What is counter-current flow in fish gills?
It is the opposite-direction movement of water and blood at the gill exchange surface. This arrangement helps maintain an oxygen gradient along the lamellae, supporting effective gas exchange.
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