Salt-water and fresh-water fish both maintain similar blood osmotic pressures, and NaCl concentrations, which are also similar to humans. Explain the ionic and osmotic challenges faced by salt and freshwater fish, and how they manage to homeostatically maintain blood ionic concentrations and osmotic pressures.
What will be an ideal response?
Ans: Freshwater fish face the problem that water tends to diffuse into them, and ions tend to diffuse out. To overcome this problem, freshwater fish excrete lots of very dilute urine, eliminating the excess water they tend to gain. They overcome ion loss by actively absorbing ions, mostly across the gills. Saltwater fish face the opposite problem, with water tending to diffuse out and ions in. To overcome the water loss problem, they drink lots of saltwater. Though they gain water, they then must deal with the many ions gained. These are actively excreted, again mostly across the gills.
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