Describe the ventilation mechanisn of afish?

Question

how it breathes in and out when it's in water.

Answer 1

Fish take in a mouthful of water and let the water pass over their gills. The gills extract the oxygen from the water. This oxygen is distributed to the organs of the fish to maintain their proper functioning. The fish breaths out by simply passing the leftover carbon dioxide out of the gills and back into the water.

Feel free to ask more detailed questions about the structure/function of gills if you need to!