What fraction of extracellular fluid is interstitial?

• A. 25%
• B. 50%
• C. 75%
• D. 100%

Answer: (C)

Extracellular fluid is split into two main parts: interstitial fluid in the tissue spaces and plasma inside blood vessels. Interstitial fluid makes up about three quarters of extracellular fluid, with plasma making up about the remaining quarter. So, interstitial fluid is roughly 3/4 of extracellular fluid, i.e., about 75%. (For context, total body water is about one-third extracellular and two-thirds intracellular; within that extracellular portion, the 75% interstitial/25% plasma split is the standard ratio.)