Interstitium accounts for what fraction of extracellular fluid?

• A. 1/4
• B. 1/3
• C. 1/2
• D. 3/4

Answer: (D)

Fluid outside cells is split into two main compartments: plasma (the fluid inside blood vessels) and interstitial fluid (the fluid in tissue spaces around cells). The interstitial compartment makes up about three quarters of the extracellular fluid, while plasma accounts for roughly one quarter. So the interstitium comprises about 3/4 of extracellular fluid.

This distribution reflects how fluid moves between vessels and surrounding tissues: most of the extracellular fluid is in the tissue spaces, with a smaller portion remaining in the intravascular space. Lymphatic drainage helps return excess interstitial fluid to circulation, maintaining this balance. In typical terms, if extracellular fluid is around 14 L, about 10.5 L is interstitial and about 3.5 L is plasma.

So the interstitium accounts for roughly three quarters of extracellular fluid.