Which fluid type is most effective at expanding the extracellular fluid (ECF) compartment?

• A. Crystalloid
• B. Colloid
• C. Hypertonic solution
• D. Hypotonic solution

Answer: (B)

Colloids are large-molecule fluids that stay mainly in the vascular (plasma) space, which is a major component of the extracellular fluid. Infusing colloids increases oncotic pressure in the blood, pulling water from the interstitial and intracellular spaces into the vessels. That osmotic pull expands the plasma volume effectively, so the extracellular fluid compartment grows because its plasma portion expands and water is drawn into it from surrounding tissues.

In comparison, crystalloids spread more evenly across the extracellular fluid, with a substantial portion moving into the interstitial space, so the intravascular (plasma) expansion is less pronounced for the same volume infused. Hypertonic solutions can transiently draw water from inside cells into the extracellular space to boost vascular volume, but their effects are more temporary and more tightly regulated. Hypotonic solutions encourage water movement into cells, reducing extracellular fluid.