Gills and Countercurrent Exchange
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Gills exchange gases across thin, blood-filled filaments bathed by water; many fish maximize uptake through countercurrent exchange, blood and water flowing opposite directions to sustain the gradient.
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In a lab, measuring oxygen extraction along a perfused gill confirms uptake stays high across its entire length.
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Parallel flow would let the gradient equalize partway through, wasting much of the gill's surface.