How much TBW is contained in the intracellular fluid?

• A. Approximately 1/3 of total body water
• B. Approximately 2/3 total body water
• C. Approximately 1/2 of total body water
• D. Approximately 3/4 of total body water

Answer: (B)

The idea being tested is how total body water is distributed between the two major compartments: intracellular and extracellular. In a typical adult, total body water is about 60% of body weight. Of this, roughly two-thirds resides inside cells (intracellular fluid) and about one-third is outside cells (extracellular fluid). For a 70 kg person, that’s about 42 L of TBW, with roughly 28 L being intracellular and about 14 L extracellular (plasma ~3 L and interstitial ~11 L). So intracellular fluid is approximately two-thirds of TBW. The other proportions would under- or overestimate this distribution.