Which electrolyte regulates extracellular fluid and helps control stomach pH?

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

Answer: (C)

Chloride is the extracellular anion that helps regulate extracellular fluid by maintaining electroneutrality with sodium and contributing to the osmotic balance of the plasma. In the stomach, chloride ions pair with hydrogen ions to form hydrochloric acid, which is the primary determinant of gastric pH. This direct role in forming stomach acid explains why chloride best fits the description. Potassium is largely intracellular and supports cellular excitability; sodium governs overall extracellular fluid volume and osmolality but doesn’t directly set stomach pH. Bicarbonate serves as a major buffer in blood, balancing systemic pH, but the stomach’s acidity comes from hydrochloric acid formed with chloride.