Mammals, including humans, have air inhaling lungs, and they must be dry and void of fluids to function properly. When humans take a breath, small air sacs in our lungs pull oxygen out of the air and carry it to our body’s cells. Thus, the lungs of mammals would not function very well for a fish, as one lungful underwater would fill them up with fluid and make them unusable.
Nevertheless, fishes need oxygen to breathe, too. To take away oxygen from the water, they rely on distinctive organs termed “gills.”
Gills are fluffy organs jam-packed with blood vessels. A fish breath by swallowing water through its mouth and pushing it out through the gill openings. As water washes through the thin walls of the gills, oxygen dissolves and moves into the blood and travels to the fish’s cells.
Gills take the oxygen out of the water
Now you ask, If fishes can respire underwater, then why do some fishes, like whales and dolphins come to the surface of the ocean? Because whales and dolphins aren’t fish at all! They are mammals, just like humans.