| Gills usually consist of a thin filament tissue, lamellae
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| Tufted processes that have highly folded surfaces to increase surface area
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| The delicate nature of gills is possible because the surrounding water provides
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| support
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| The blood or other body fluid must be in it to make contact with the
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| respiratory surface for ease of diffusion
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| The high surface area is crucial to the gas exchange of aquatic organisms
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| Water only contains a small fraction of oxygen that air does
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| Water is 777 times more dense than air and is 100 times more viscous
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| Oxygen has a diffusion rate in air 10,000 times greater than in water
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| The density of water prevents the gills from collapsing and lying on top of
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| each other
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| Which is what happens when a fish is taken out of water
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| Usually water is moved across gills in one direction like
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| When a fish breathes, it draws a mouthful of water at regular intervals
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| Then it draws the sides of its throat together, forcing the water through the
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| gill openings
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| So that it passes over the gills to the outside
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| The gills of vertebrates typically develop in the walls of the pharynx,
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| Along a series of gill slits opening to the exterior
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| Most species employ a counter-current exchange system to enhance the diffusion
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| of substances in and out of the gill
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| With blood and water flowing in opposite directions to each other
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| The gills are composed of comb-like filaments, the gill lamellae,
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| which help increase their surface area for oxygen exchange |