In healthy adults, which compartment contains the largest share of total body water?

• A. Intracellular fluid
• B. Extracellular fluid
• C. Intravascular fluid
• D. Interstitial fluid

Answer: (A)

Most body water is inside cells. In an adult, total body water is about 60% of body weight, and roughly two-thirds of that water is intracellular fluid, the fluid contained within cells. The remaining one-third is extracellular fluid, which sits outside cells and includes interstitial fluid (the fluid between cells) and intravascular fluid (plasma). Within extracellular fluid, interstitial fluid makes up the larger share, while plasma is a smaller portion. So, the compartment with the largest share of total body water is the intracellular fluid.