What fraction of total body water is intracellular?

• A. 1/3
• B. 2/3
• C. 3/4
• D. 1/2

Answer: (B)

The idea here is how body water is split between inside and outside cells. In a typical adult, total body water is about 60% of body weight. Of that water, roughly two-thirds is inside cells (intracellular fluid) and one-third is outside cells (extracellular fluid). So the fraction of total body water that is intracellular is 2/3. For example, a 70 kg person has about 42 liters of TBW, and about 28 liters of that is intracellular fluid, with the remaining ~14 liters extracellular. This distribution reflects the cellular environment and the ion concentrations that keep fluids partitioned between compartments.