Which cation is most abundant in the extracellular fluid?

Which cation is most abundant in the extracellular fluid?

• A. Potassium
• B. Calcium
• C. Sodium
• D. Magnesium

Answer: (C)

Sodium dominates the extracellular fluid as the principal cation, and its concentration largely sets the osmolarity and volume status of the space outside cells. This makes it the most abundant positively charged ion in the extracellular compartment. Potassium is the main cation inside cells, so its extracellular level is much lower. Calcium does exist outside cells and is crucial for signaling, muscle contraction, and clotting, but its overall concentration in the extracellular fluid is much smaller than sodium’s. Magnesium is present too, but in smaller amounts compared with sodium. So, sodium is the leading extracellular cation by both abundance and influence on fluid balance.

Sodium dominates the extracellular fluid as the principal cation, and its concentration largely sets the osmolarity and volume status of the space outside cells. This makes it the most abundant positively charged ion in the extracellular compartment. Potassium is the main cation inside cells, so its extracellular level is much lower. Calcium does exist outside cells and is crucial for signaling, muscle contraction, and clotting, but its overall concentration in the extracellular fluid is much smaller than sodium’s. Magnesium is present too, but in smaller amounts compared with sodium. So, sodium is the leading extracellular cation by both abundance and influence on fluid balance.