Extracellular fluid is rich in which ion?

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

Answer: (C)

Extracellular fluid is dominated by the cation sodium. The Na+/K+ ATPase pumps sodium out of cells and potassium in, keeping a high Na+ concentration outside and a high K+ concentration inside. This sodium-rich environment helps set osmotic balance and drives much of the extracellular fluid’s behavior, including influences on nerve and muscle function. While chloride is also abundant outside as the main anion to balance charge, and calcium is present but at much lower free concentrations, the defining ion of the extracellular fluid is sodium.