The movement of water among body compartments is strongly affected by which mineral?

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

Answer: (D)

Water movement between body compartments is driven by osmotic gradients, and the main determinant of these gradients in the extracellular space is sodium. Sodium sits as the major extracellular cation and largely sets the osmolarity of the extracellular fluid. When extracellular sodium rises, the extracellular osmolarity increases, pulling water out of cells to balance the osmotic pressure; when sodium falls or excess water accumulates, water shifts back into cells to restore balance. Hormonal mechanisms like ADH and mechanisms controlling sodium reabsorption in the kidneys help maintain this delicate balance.

Potassium, while essential and largely intracellular, affects cell volume within its own compartment but does not provide the primary osmotic driver for movement of water between compartments. Magnesium and calcium have critical roles in signaling and other functions, but they do not dictate the same direct osmotic water shifts that sodium does.

So the mineral most strongly shaping water distribution among compartments is sodium.