What fraction of total body water is extracellular fluid?

• A. One-third.
• B. Two-thirds.
• C. Half.
• D. All body fluid.

Answer: (A)

Understanding how body water is distributed between intracellular and extracellular compartments helps explain this fraction. In an average adult, total body water is about 60% of body weight. Extracellular fluid makes up roughly 20% of body weight, which is about one-third of the total body water (20% out of 60%). The remaining 40% of body weight is intracellular fluid. Extracellular fluid itself consists of interstitial fluid and plasma (with a small amount of transcellular fluid). So, the fraction of total body water that is extracellular fluid is about one-third. This ratio can vary a bit with age or body composition, but one-third is the typical value.