How do fish breathe underwater without lungs?

Fish breathe underwater using gills instead of lungs. As a fish swims, it takes water in through its mouth and passes it over the gills, which are made up of thin, feathery filaments rich in tiny blood vessels. Oxygen dissolved in the water diffuses across the gill membranes directly into the bloodstream, while carbon dioxide moves in the opposite direction and is expelled back into the water. The water then exits through the gill openings on the sides of the fish's head. This countercurrent exchange system, where blood flows opposite to the direction of water, makes the process highly efficient, allowing fish to extract up to 80 percent of the available oxygen from the water passing through their gills.