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Fish breathe using gills. Gas exchange occurs in a structure of the gills called the gill fillaments. Blood in the fillaments flows in the opposite direction of water, this is called a counter current system. Some fish will swim against water currents to increase the amount of oxygen they are getting. There are also some fish that, in addition to the gills, can gulp air and the oxygen diffuses in the fishes swim bladder.

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13y ago

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