<P> 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 8 cm per litre compared to 210 in the same volume of air . Water is 777 times more dense than air and is 100 times more viscous . Oxygen has a diffusion rate in air 10,000 times greater than in water . The use of sac - like lungs to remove oxygen from water would not be efficient enough to sustain life . 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 . '' </P>
<P> 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 8 cm per litre compared to 210 in the same volume of air . Water is 777 times more dense than air and is 100 times more viscous . Oxygen has a diffusion rate in air 10,000 times greater than in water . The use of sac - like lungs to remove oxygen from water would not be efficient enough to sustain life . 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 . '' </P>
where does gas exchange take place in the gills
where does gas exchange take place in the gills
surface area
surface area