If extracellular fluid is 16 L and intravascular space is 4 L, what is the interstitial fluid volume?

• A. 8 L
• B. 12 L
• C. 4 L
• D. 16 L

Answer: (B)

Extracellular fluid is the sum of two main compartments: the intravascular space (plasma) and the interstitial fluid that surrounds cells. To get the interstitial volume, subtract the intravascular (plasma) portion from the total extracellular fluid. Here, 16 L of extracellular fluid minus 4 L of intravascular fluid leaves 12 L for the interstitial space. This makes sense because the interstitial space is the remaining part of the extracellular fluid not contained within blood vessels. The other values would mix the compartments differently: 4 L would be just the plasma, 16 L would ignore the division into plasma and interstitial, and 8 L wouldn’t account for the known plasma volume.