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Why Donít Fish Drown?
When you watch a goldfish gulping water, rhythmically opening and closing its mouth, you are seeing it breathe. Fish take oxygen from water rather than air. Instead of drowning from water, they require it for life.
From the mouth, water passes over the fishís gills, which are fleshly filaments with blood vessels lying close to the surface. In the gills carbon dioxide, which has been carried away from body cells by the blood, is exchanged for oxygen dissolved in water. Gills function so efficiently that they can take up about 75 percent of available oxygen.
How much oxygen is in the water influences which fish live there. Because trout need large amounts of oxygen, they inhabit cold water, which holds more oxygen than warm water does. In contrast, carp and catfish thrive where the oxygen content is low in sluggish streams, for example.
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