Fish gills exchange gases between water and blood: water moves across thin, blood-rich gill surfaces, dissolved oxygen diffuses into the blood, and carbon dioxide diffuses into the water. In many fish, water and blood flow in opposite directions across the exchange surface, helping preserve the oxygen gradient that drives diffusion.
Gill arches support filaments bearing numerous thin lamellae. Fish ventilate these surfaces in different ways, and gills also help regulate salts and ions and excrete certain wastes.
What are the main parts of a fish gill?
Gill arches support the respiratory structures, and filaments extending from the arches bear numerous lamellae. The thin, blood-rich lamellae bring water and blood close together, allowing gases to diffuse between them.
| Gill part | What it is | What it does |
|---|---|---|
| Gill arch | A supporting structure | Supports gill structures and may bear gill rakers |
| Gill raker | A projection associated with a gill arch | Helps with feeding; it is not the primary gas-exchange surface |
| Gill filament | A branch extending from a gill arch | Bears numerous lamellae |
| Lamella | A thin exchange surface on a filament | Brings water and blood close together for gas diffusion |
How do fish gills exchange oxygen and carbon dioxide?
Fish use oxygen gas dissolved in water, not oxygen chemically bound inside water molecules. As water flows over the lamellae, oxygen diffuses across the thin surface into nearby blood. Blood carries that oxygen to the fish’s tissues. Carbon dioxide produced by the tissues travels in the blood to the gills and diffuses into the water. Washington State University’s explanation of fish breathing describes this exchange and the role of countercurrent flow.
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The path of gill respiration
- Water enters the fish’s mouth or otherwise reaches the gills, depending on the fish’s ventilation strategy.
- Water moves across the gill filaments and lamellae.
- Dissolved oxygen diffuses from the water into blood in the lamellae.
- Blood carries oxygen to the fish’s tissues.
- Blood returns carrying carbon dioxide produced by the tissues.
- Carbon dioxide diffuses from blood into water, which then flows away from the gills.
Why does blood flow opposite to water in fish gills?
In countercurrent exchange, blood and water flow in opposite directions across the lamellae. As water loses oxygen along the exchange surface, it continues to meet blood with a lower oxygen level, so oxygen can keep diffusing into the blood. This opposing flow helps maintain a gradient over more of the surface than same-direction flow would. The University of Tennessee’s countercurrent-system resource explains this gradient.
How do fish move water over their gills?
Many bony fish use buccal pumping: coordinated movements of the mouth and gill-cover chamber move water across the gills and toward the opercular opening. Some fast-swimming fish, including sharks and tunas, can use ram ventilation, in which swimming helps drive water over the gills. These are examples of different strategies, not a rule that every fish must swim continuously. The University of Hawaiʻi at Mānoa describes these fish structures and ventilation methods.
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What else do fish gills do besides exchange gases?
Gills also have roles in osmoregulation—the control of water and salts—and in ion balance and excretion. Freshwater and marine fish face different salt and water-regulation challenges, and species do not all use identical mechanisms. The University of Wisconsin–Madison’s explainer discusses these additional gill functions.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Frequently Asked Questions
Do fish get oxygen by breaking apart water molecules?
No. Fish use oxygen gas already dissolved in water. It diffuses across the gill lamellae into the blood.
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Why do fish need water to flow over their gills?
Water flow supplies dissolved oxygen to the exchange surfaces and carries away carbon dioxide. The thin lamellae and nearby blood allow the gases to diffuse between water and blood.
Do all fish breathe through their gills in the same way?
No. Many bony fish pump water with coordinated mouth and gill-cover movements, while some swimmers use ram ventilation. Ventilation strategies vary among fish.
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What is countercurrent exchange?
It is the opposing movement of water and blood across the gill exchange surface. This arrangement helps keep an oxygen gradient available along the lamellae so oxygen can continue diffusing into the blood.
Quick Recap
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