All animals need oxygen. We get oxygen from the air we breathe. How do fish get theirs?
Fish obtain oxygen from the water they live in rather than from the air. They have a specialized respiratory system that allows them to extract dissolved oxygen from water.
Fish have gills, which are specialized organs for respiration. Gills contain specialized structures called filaments, which have numerous thin and delicate projections known as lamellae. As fish swim, water passes through the mouth and over the gills. The gills extract the dissolved oxygen from the water and release carbon dioxide.
Inside each gill filament, there are tiny blood vessels called capillaries. The capillaries have a thin layer of cells, which exchange oxygen from the water with carbon dioxide from the fish's blood. This process is known as counter-current exchange, where water and blood flow in opposite directions. This efficient mechanism ensures maximum oxygen can be absorbed from the water.
Therefore, fish obtain their necessary oxygen by extracting dissolved oxygen through their gills from the water they live in.