Which ion maintains extracellular fluid osmolarity?

Which ion maintains extracellular fluid osmolarity?

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

Answer: (A)

Sodium governs extracellular fluid osmolarity. Osmolarity is determined by the number of osmotically active particles in a solution, and in the extracellular space, sodium salts (mainly with chloride and bicarbonate) provide the vast majority of those particles. Because sodium is present at high extracellular concentrations, it sets the osmotic pressure that determines how water moves between compartments. Potassium is mostly inside cells, so it doesn’t drive extracellular osmolarity as much. Calcium and magnesium are present in smaller amounts extracellularly, so their contribution to overall osmolarity is relatively small, though they have other important roles in physiology.

Sodium governs extracellular fluid osmolarity. Osmolarity is determined by the number of osmotically active particles in a solution, and in the extracellular space, sodium salts (mainly with chloride and bicarbonate) provide the vast majority of those particles. Because sodium is present at high extracellular concentrations, it sets the osmotic pressure that determines how water moves between compartments. Potassium is mostly inside cells, so it doesn’t drive extracellular osmolarity as much. Calcium and magnesium are present in smaller amounts extracellularly, so their contribution to overall osmolarity is relatively small, though they have other important roles in physiology.