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Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →Fish usually get the oxygen needed for cellular respiration from water flowing across their gills. Oxygen diffuses into blood at the gills, the circulation carries it to body tissues, and cells use it—mainly in mitochondria—to produce ATP, the immediately usable form of cellular energy. Carbon dioxide made by this metabolism returns in the blood to the gills and diffuses into the water.
From water to cellular energy
In most fish, the process follows this sequence:
- Water moves over the gills.
- Oxygen crosses the gill surface into the blood, while carbon dioxide leaves the blood.
- The circulatory system delivers oxygenated blood to tissues.
- Cells use oxygen in mitochondria during cellular respiration and produce ATP.
- Carbon dioxide produced by the cells is carried back to the gills and released into the water.
1. Water flows across the gills
Most fish obtain oxygen from dissolved oxygen in water rather than from the air. Gills contain many filaments and thin exchange surfaces supplied with blood vessels. Their folded structure creates a large area through which gases can move.
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Ventilation—the movement of water across the gills—may be produced by the fish’s swimming movements, by movements of the mouth and gill covers, or by both, depending on the fish. Ventilation brings fresh, oxygen-containing water to the respiratory surface; it is not itself cellular respiration.
2. Oxygen and carbon dioxide are exchanged by diffusion
Blood arriving at the gills has relatively little oxygen and carries carbon dioxide from the tissues. When oxygen concentration is higher in the surrounding water than in that blood, oxygen diffuses across the thin gill surface into the capillaries. Carbon dioxide diffuses in the opposite direction, from blood into the water.
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This is gas exchange. The gills perform this exchange, but they do not carry out cellular respiration.
3. The circulation transports oxygen to tissues
In the usual fish circulation, the heart pumps oxygen-poor blood to the gills. After it becomes oxygenated, the blood travels through the body to organs and other tissues, where cells can use the oxygen. It then returns to the heart, completing a single circuit through the gills and body. Fish generally have a two-chambered heart in this pattern.
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| Stage | What happens | Where it occurs |
|---|---|---|
| Ventilation | Water is moved over the respiratory surface | Mouth, gill chamber and gills |
| Gas exchange | Oxygen enters blood and carbon dioxide leaves it by diffusion | Thin gill exchange surfaces and capillaries |
| Transport | Blood distributes oxygen and carries carbon dioxide | Heart, blood vessels and circulation |
| Cellular respiration | Cells use oxygen to release usable energy as ATP | Body cells, largely in mitochondria |
4. Cells use oxygen to make ATP
Once oxygen reaches a tissue, it enters cells and is used in cellular respiration. The mitochondria are the main sites identified for this energy-releasing process. Energy from the cell’s food molecules is captured in ATP, which powers cellular work such as movement, growth, repair and maintaining internal conditions.
Thus, a fish does not perform cellular respiration “in its gills.” The gills obtain and exchange gases; the cells throughout the body use the delivered oxygen to produce ATP.
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5. Carbon dioxide returns to the water
Cellular respiration produces carbon dioxide as a waste product. Carbon dioxide moves from cells into the blood, which transports it back to the gills. Because its concentration is higher in the returning blood than in the surrounding water, it diffuses across the gill surface and leaves the fish.
Do all fish respire in exactly the same way?
Gills are the primary aquatic respiratory organs for most fish, but species are not anatomically identical. Some air-breathing fish also use accessory structures, such as a modified swim bladder, skin or another specialized organ, to take oxygen from air. In those species, the oxygen source and respiratory surfaces can differ from the common gills-only pattern, although cells still use oxygen in cellular respiration and produce ATP.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Breathing, gas exchange and cellular respiration: the difference
- Breathing or ventilation: moving water (or, for an air-breathing fish, air) across a respiratory surface.
- Gas exchange: diffusion of oxygen into blood and carbon dioxide out of blood at a respiratory surface.
- Cellular respiration: the chemical energy-releasing process inside cells that uses delivered oxygen and produces ATP.
These are connected stages, but they are not interchangeable terms. A fish can ventilate its gills, exchange gases there, transport oxygen in blood and then carry out cellular respiration in its tissues.
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