Intravascular space is part of which TBW compartment?

• A. Intracellular
• B. Extracellular
• C. Transcellular
• D. Interstitial

Answer: (B)

Intravascular space is part of the extracellular fluid. Total body water is split into intracellular fluid (inside cells) and extracellular fluid (outside cells). The extracellular fluid is then divided into interstitial fluid (the fluid between cells), plasma within blood vessels (the intravascular space), and a smaller transcellular component (fluids like CSF, synovial, etc.). Since the intravascular space contains plasma inside the vessels, it belongs to the extracellular compartment, not the intracellular or interstitial compartments (and not the transcellular component, which is a separate subset). In roughly terms, two-thirds of TBW is intracellular, and about one-third is extracellular, with the extracellular portion further divided so that plasma is the intravascular part and interstitial fluid makes up the majority of the remaining extracellular fluid.