Which extracellular ion is primarily responsible for ventricular depolarization and helps regulate water balance?

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

Answer: (B)

Sodium is the primary extracellular ion driving ventricular depolarization. The rapid upstroke of the ventricular action potential occurs as sodium ions rush into cardiac muscle cells through voltage-gated sodium channels, producing the sharp depolarization that activates the ventricles. Sodium also plays a key role in regulating water balance and extracellular fluid volume by influencing osmolality and fluid distribution in the body. Calcium, while crucial for contraction and contributing to the plateau phase of the action potential, is not the main driver of the initial depolarization in the ventricles. Potassium governs resting membrane potential and repolarization, and magnesium acts as a modulator but is not the principal depolarizing ion. Therefore, sodium best fits both aspects of the question.