Since fish live in a watery environment, they do not have lungs that function like those of higher vertebrates. Fish usually have gills, surfaces over which water flows that are vascularized and allow gas exchange. Water can dissolve far less oxygen than air, however, and fish are able to survive with this low oxygen concentration for two major reasons. Firstly, in general, the metabolic rates of fishes are much lower than those of higher vertebrates, so they require less oxygen. Secondly, fishes have developed a countercurrent system in which the blood passing through the gills travels in a direction opposite to the flow of water over the gills. This allows for the extraction of almost all the available oxygen from the water. Fish have different ways of moving water over their gill surfaces; many have a flaplike operculum which slowly moves water across the gills; others use their mouths to gulp water, or swim quickly with their mouths open to facilitate higher oxygen intake. Only when animals made the transition from water to land was it practical for the modern lung to evolve.
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