Which physiological challenge necessitates countercurrent exchange in fish swim bladders when increasing depth?

💡 Explanation

When fish descend, increasing hydrostatic pressure raises blood and swim bladder partial pressure increasing the driving force for oxygen diffusion out of the blood according to Henry's law;countercurrent exchange maximizes oxygen transfer against increasingly steep concentration increase preventing deflation. Therefore oxygen diffusion limitation necessitates countercurrent exchange, rather than narcosis, regulation, or carbon dioxide which are separate unrelated biological challenges.

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