Gills extract O2 gas that has dissolved in the water. Fish are cold blooded and do not need a high amount of oxygen to function. Gills operate like lungs, except that instead of one opening, gills absorb oxygen from a flow of water moving past its surface area in one direction. Like lung tissue, gills consist of small blood vessels with thin walls that oxygen can permeate.
Gills are external respiratory organs of most aquatic animals and in fish are located in gill chambers at the rear of the mouth. To absorb the oxygen, water is taken in from the mouth and is forced through opening called gill slits and then passes through the gill clefts. Each gill is composed of numerous threadlike gill filaments containing capillaries enclosed in a thin membrane. Oxygen is absorbed by these capillaries from the passing water and carbon dioxide is discharged. The gill rakers, bony comblike projections, strain solid material from the water preventing it from passing through the gill slits and going down the esophagus.
The process is simply diffusion. Very well detailed in the below link.
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