where does gas exchange take place in the gills
Fish gill Gills usually consist of thin filaments of tissue, branches, or slender tufted processes that have a highly folded surface to increase surface area. The high surface area is crucial to the gas exchange of aquatic organisms as water contains only a small fraction of the dissolved oxygen that air does. A cubic meter of air contains about 250 grams of oxygen at STP. The concentration of oxygen in water is lower than air and it diffuses more slowly. In a litre of freshwater the oxygen content is 8Â cm3 per litre compared to 210 in the same volume of air.[8] Water is 777 times more dense than air and is 100 times more viscous.[8] Oxygen has a diffusion rate in air 10,000 times greater than in water.[8] The use of sac-like lungs to remove oxygen from water would not be efficient enough to sustain life.[8] Rather than using lungs "Gaseous exchange takes place across the surface of highly vascularised gills over which a one-way current of water is kept flowing by a specialised pumping mechanism. The density of the water prevents the gills from collapsing and lying on top of each other, which is what happens when a fish is taken out of water."[8]