Final answer:
The principal cation in intracellular fluid is potassium, which is maintained at high levels inside cells by the sodium-potassium pumps and plays a critical role in the cellular functions
Step-by-step explanation:
The principal cation in intracellular fluid is potassium. Potassium is crucial for maintaining the resting membrane potential in neurons and muscle fibers and plays a minimal role in the osmotic pressure compared to other cations. In contrast, sodium ions are found in higher concentrations in the extracellular fluid, where they play a significant role in osmotic pressure along with chloride ions.
While calcium and magnesium are also present as cations in the intracellular fluid, their concentrations are lower than that of potassium. The high levels of potassium and low levels of sodium within the intracellular fluid are maintained by the sodium-potassium pumps in cell membranes, which actively transport sodium out of the cell and potassium into the cell, using ATP for energy.