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What matters is that water is circulating over the thin membranes of the gill filaments, so the CO2, which has a higher concentration than the surrounding water, can diffuse out. The oxygen, which has a higher concentration in the water than in the fish, diffuses in. Shouldn't really matter much which direction the fish is getting pulled in. I'll get back when I check my notes, I vaguely remember something about the way the blood vessels are arranged for maximum oxygen absorption.
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