Which of the following statements is false about the electrolyte chloride?

• A. Main anion of the extracellular fluid
• B. Can shift from the extracellular plasma to the intracellular fluid of red blood cells
• C. Unable to be reabsorbed by active transport
• D. Measured in serum, urine, and sweat

Answer: (C)

Chloride moves across body compartments and is actively handled by the kidneys, not locked into passive movement only. It is the main extracellular fluid anion and works with sodium to maintain osmotic balance and electroneutrality. In red blood cells, chloride shifts in and out (the chloride shift) as bicarbonate moves, helping CO2 transport while preserving charge balance. In the kidney, chloride reabsorption occurs through active transport mechanisms, such as the Na+-K+-2Cl- cotransporter in the thick ascending limb and other transporter and channel systems along the nephron, often linked to the Na+/K+ ATPase gradient. Because of this active reabsorption capability, the statement that chloride cannot be reabsorbed by active transport is false. It’s also routinely measured in serum, urine, and sweat, with sweat testing used in certain clinical contexts.