What is the major extracellular cation?

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

Answer: (A)

Sodium is the major extracellular cation. It is the most abundant positively charged ion in extracellular fluid, and its high concentration helps determine extracellular osmolality and fluid distribution between body compartments. Sodium also powers nerve and muscle functions through depolarization, making it central to signaling and overall cellular activity.

Chloride, by contrast, is the main extracellular anion, balancing the positive charges of sodium. Magnesium and calcium are also extracellular cations, but their concentrations are much lower than sodium’s, and calcium is often tightly regulated and largely bound in the blood. Because of these much lower levels, they do not dominate the extracellular cation pool the way sodium does.