Crystalloids mimic the composition of which body fluid compartment?

• A. Intracellular fluid
• B. Extracellular fluid
• C. Plasma
• D. Interstitial fluid

Answer: (B)

Crystalloids are solutions with small solute particles that freely cross capillary walls, so they quickly equilibrate with the extracellular space rather than staying inside cells. That makes their electrolyte composition resemble the extracellular fluid, which includes both the interstitial fluid and plasma as a whole. Because they closely match extracellular fluid, crystalloids are used to expand extracellular volume. They distribute rapidly from the intravascular space into the interstitial space, with only a portion remaining in the plasma, which aligns with the idea of mimicking the extracellular compartment rather than the intracellular one. Intracellular fluid, by contrast, has much higher potassium and different solute composition, so crystalloids don’t mirror that. Plasma is part of the extracellular space but contains proteins and other constituents not present in crystalloids, so crystalloids are described as mimicking the extracellular fluid in general rather than plasma specifically.