What fraction of extracellular fluid is located in the intravascular space?

• A. 1/3
• B. 1/4
• C. 3/4
• D. 1/2

Answer: (B)

Extracellular fluid is the fluid outside cells and is split into two main parts: plasma inside the blood vessels and interstitial fluid in the tissue spaces. Of the extracellular fluid, about a quarter resides in the intravascular space (plasma) while roughly three quarters is in the interstitial space. So the fraction of extracellular fluid located in the intravascular space is one quarter. For context, total body water is about 60% of body weight, extracellular fluid is about one third of that, and the intravascular portion is about one quarter of the extracellular fluid.