Biology / Unit 13 Bio 3147 short lesson, about 20 seconds
Gills and Countercurrent Exchange
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The idea
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.
In practice
In a lab, measuring oxygen extraction along a perfused gill confirms uptake stays high across its entire length.
Where people slip
Parallel flow would let the gradient equalize partway through, wasting much of the gill's surface.
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