How much Na+ is intracellular?

• A. 1%
• B. 5%
• C. 10%
• D. 20%

Answer: (C)

Intracellular Na+ stays low because the Na+/K+ ATPase pump actively exports Na+ from the cell in exchange for K+. This keeps the inside of the cell much less salty than the outside, which is essential for the membrane potential and cell volume control.

In typical physiology, extracellular Na+ is about 140 mEq/L, while intracellular Na+ is around 10 mEq/L. If you think in terms of total body Na+, roughly 10% resides inside cells and about 90% remains in the extracellular fluid. That makes about ten percent the best match to the question.

So the correct amount is about 10% of Na+. The other options don’t fit with the commonly observed intracellular Na+ level and the strong outward pumping that maintains the gradient.