How Do Gills Work?


Quick Answer

Gills work in fish in much the same way as lungs do in mammals, as they help take oxygen into the blood and release carbon dioxide. Fish draw water into their mouths, which then travels past the many tiny blood vessels in the gills that absorb oxygen from the water.

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Full Answer

The blood vessels in the gills are incredibly thin, allowing oxygen from the water to absorb into the bloodstream and carbon dioxide to be released through the same mechanism. All fish gills have a large surface area with thousands of small capillaries to extract as much oxygen from the water as possible. This is much harder for fish than for land animals, as water typically only contains between 4 and 8 parts of oxygen for every 1 million particles, whereas air has around 200,000 molecules of oxygen per million.

After a fish takes water into its mouth, it reduces the pressure in its mouth to force the water to pass through the gill filaments. Oxygen is absorbed and carbon dioxide is released into the water as it passes through the gills, and then the water exits the fish through the operculum.

The biggest problem with breathing this way is that the number of exposed blood vessels makes it hard for the fish to control the level of salt in their bodies. Freshwater fish continually lose a large amount of salt through this process, while saltwater fish struggle to keep their bodies from absorbing too much salt from the water.

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