Tissues of sharks are isotonic to seawater,but their concentrations of sodium ions,potassium ions,and chloride ions in cells and extracellular fluids are similar to those of freshwater fishes.What can you infer about the movement of sodium and chloride in these animals?

Question

Quizplus · Accepted Answer

Since the concentrations of sodium and chloride in the tissues of sharks are isotonic to seawater, it suggests that these ions are able to diffuse freely across the gills from seawater into the shark's blood. The fact that the concentrations of these ions in the cells and extracellular fluids are similar to those of freshwater fishes implies that the shark must somehow actively conserve these ions and limit excretion, perhaps by reabsorbing them from the urine before it is excreted. However, the initial influx of sodium and chloride into the shark's body likely occurs through diffusion across the gills.

Tissues of Sharks Are Isotonic to Seawater,but Their Concentrations of Sodium