Which of the following describes the distribution of sodium and potassium between cells (ICF) and body fluids (ECF)?

The correct answer describes the physiological roles and distribution of sodium and potassium within the body's fluid compartments. Sodium (Na+) predominantly exists in the extracellular fluid (ECF), while potassium (K+) is primarily found inside the cells, or intracellular fluid (ICF).

This distribution is essential for maintaining cellular function, resting membrane potential, and is critical for processes like nerve impulse transmission and muscle contraction. The sodium-potassium pump actively moves sodium out of the cells and potassium into the cells, thus establishing the necessary concentration gradients. As a result, Na+ is maintained at high levels outside cells, and K+ is retained at higher concentrations within the cells.

The other options misrepresent these distributions. The assertion that sodium is mainly in cells and potassium is in body fluids is contrary to how these electrolytes function in the body. Stating that sodium is equal in both compartments ignores the critical role of active transport mechanisms that maintain their respective concentrations. Finally, the notion that potassium has no significant presence in body fluids overlooks its essential presence in the extracellular space, although in much lower concentrations compared to its levels inside the cells.