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As their main gas exchange organ, fish use ( gills, lungs, spiracles, capillaries ). Their structure consists of ( an arch, a loop, a spiral, filaments ) from which ( filaments, arches, alveoli, spiracles ) project. Each filament contains many ( lamellae, grana, stomata ), through which capillaries run. This is the site at which gas exchange takes place. The blood flows through the capillaries in the ( opposite, same ) direction as/to the water that flows through the gills, creating a ( countercurrent, current, anti-current, monocurrent ) exchange. This ensures there is a favourable gradient in ( water potential, osmotic pressure, oncotic pressure, water flow ) between the water and the blood.
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