Which of the following is not a listed TBW compartment?

• A. Intracellular, Extracellular, Transcellular
• B. Plasma
• C. Interstitium
• D. Blood

Answer: (B)

Total body water is divided into intracellular fluid and extracellular fluid. The extracellular fluid is further broken down into interstitial fluid, plasma (the fluid within blood vessels), and transcellular fluid. Because TBW compartments refer to the larger fluid spaces themselves, plasma is not listed as a separate TBW compartment—it is the intravascular portion of extracellular fluid. Blood contains plasma plus cellular elements, so it isn’t a TBW compartment on its own either. Therefore, plasma stands out as not being a listed TBW compartment.