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Fish

How do fish breathe?

How a fish breathesHow a fish breathesMost kinds of fish cannot survive out of water, because they do not have lungs to breathe air. Instead, they are able to filter oxygen from the water around them using slits in their heads called gills. To breathe, fish open their mouths and take in water that contains oxygen. When they pump the water out through slits in the sides of their heads, it passes over feathery filaments (thread-like fibres) in the gills. These filaments contain blood vessels with thin walls through which oxygen from the water passes into the blood. At the same time, waste carbon dioxide in the blood passes into the water. Bony fish usually have a hard protective flap over their gills, called the operculum. It controls the flow of water through the gills so that the maximum amount of oxygen is absorbed. 


Gills removed from a fish’s headGills removed from a fish’s head

Gills

Bony fish have a single external gill opening on either side of their heads. Most have five pairs of gills. Sharks and rays—cartilaginous fish—have a separate gill slit for each gill (between five and seven pairs). In both types of fish, each gill is supported by a gill arch and protected by gill rakers. The gill rakers help make sure that no other material gets into the gill filaments to clog them up. 

Using their gills, fish are able to extract up to 70% of the oxygen dissolved in the water.

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