Which extracellular fluid solute is a major cation outside cells and helps regulate fluid balance?

Which extracellular fluid solute is a major cation outside cells and helps regulate fluid balance?

• A. Sodium
• B. Potassium
• C. Chloride
• D. Phosphate

Answer: (A)

Sodium is the major extracellular fluid cation that sets the osmolarity of the fluid outside cells, so water movement and overall fluid balance are driven by its concentration. By maintaining extracellular osmotic pressure, sodium helps regulate blood volume and blood pressure and supports nerve impulses and muscle function. Potassium is mainly inside cells, not outside; chloride is an accompanying anion with sodium; phosphate is largely intracellular and involved in metabolism and buffering. Because sodium controls the osmotic balance that governs fluid distribution in the body, it best fits as the extracellular solute that regulates fluid balance.

Sodium is the major extracellular fluid cation that sets the osmolarity of the fluid outside cells, so water movement and overall fluid balance are driven by its concentration. By maintaining extracellular osmotic pressure, sodium helps regulate blood volume and blood pressure and supports nerve impulses and muscle function. Potassium is mainly inside cells, not outside; chloride is an accompanying anion with sodium; phosphate is largely intracellular and involved in metabolism and buffering. Because sodium controls the osmotic balance that governs fluid distribution in the body, it best fits as the extracellular solute that regulates fluid balance.