What fraction of extracellular fluid is intravascular (plasma)?

• A. 10%
• B. 25%
• C. 50%
• D. 75%

Answer: (B)

Extracellular fluid is made up of two main parts: plasma (the intravascular fluid) and interstitial fluid. The plasma portion is smaller than the interstitial portion within the extracellular space, about one-quarter of the total extracellular fluid. In numbers, total body water is roughly 60% of body weight, extracellular fluid is about 20% (around 14 L in a typical 70-kg adult), and plasma is about 5% of body weight (roughly 3.5 L). Since 3.5 L is ~25% of 14 L, plasma constitutes about one quarter of the extracellular fluid. The remaining ~75% of the extracellular fluid is interstitial. Therefore, the fraction of extracellular fluid that is intravascular (plasma) is about 25%.