Gills act similarly to the sacs in your lungs. They present a gas permeable barrier (a substance that gases can pass through but other substances, like water, can’t) to the water flowing through the gills. This allows oxygen to be absorbed directly into the blood vessels without respiration.
You can watch fish and notice that if they are not moving the flaps over their gills will be. This ensures fresh water, with oxygen, is passing through. A fish that neither moves or has moving gills will die from asphyxiation.
They don’t. There is oxygen in the water, just like there is oxygen in the air.
When you breath in the air around you your lungs don’t do anything with the nitrogen that it is mostly made up of. Similarly the fish don’t try to actually do anything with the H2O molecules that the water is mostly made up of but only filter out the oxygen that is freely accessible dissolved in the water.
All liquids and especially water contain gases from the air. Think of water as sugar and air as what is between the grains of sugar. That’s how the fizz in pop occurs. Usually the fizz is just one component of air, CO2 – in words carbondioxide gas. All the gases that make up the air we breathe will dissolve in water. The gills just extract the oxygen from the water similarly to the way our lungs extract oxygen from the air when we breathe.
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