All bony fishes must intake oxygen and get rid of carbon dioxide for their cells to function. Most bony fishes exchange gases with water using grill-like structures behind the mouth called gills. Gills are reddish because they are infused with blood vessels, which circulate oxygen through the rest of the fish. Some bony fishes swim with their mouths open, allowing water to pass through the gills, while others pump a space behind the gills to pass water over them. Because the concentration of oxygen is lower in water than air, and water is denser, lungs used underwater would not meet oxygen needs. The layers of highly folded membranes in gills create a large surface area for gas exchange. When a fish is taken from water into less dense air, its gills collapse and it gasps for oxygen. Some bony fishes have developed other, innovative ways to get oxygen, including absorption through the skin, digestive organs, or gas bladder.