Which ion has a higher concentration in extracellular fluid compared to intracellular fluid?

Calcium ions (Ca²⁺) are indeed present in higher concentrations in the extracellular fluid compared to the intracellular fluid. This disparity is crucial for various physiological processes, including muscle contraction, neurotransmitter release, and cardiac function. The significantly higher extracellular concentration of calcium is maintained by active transport mechanisms, such as the calcium pump (Ca²⁺-ATPase) and sodium-calcium exchangers, which extrude calcium from cells to prevent excessive intracellular accumulation.

In contrast, potassium ions (K⁺) are normally found at higher concentrations inside cells, where they play a vital role in maintaining the resting membrane potential and many cellular functions. Chloride ions (Cl⁻) generally have a higher concentration outside the cell but are correlated with potassium to some extent, thus not as pronounced as calcium. Amino acids are generally more concentrated inside cells where they serve as building blocks for proteins and are involved in various metabolic pathways.

This specific distribution of calcium ions is essential for initiating responses in muscles and neurons, hence highlighting calcium's distinctive role among the ions listed.