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Colloids are large molecules, such as proteins and starches, that are typically used in medical settings to expand blood volume and maintain pressure due to their oncotic properties. When colloids are infused into the body, they primarily remain within the intravascular compartment.

The intravascular compartment comprises plasma within blood vessels, where these large molecules exert oncotic pressure, helping to draw water into the vascular space. This leads to an increase in blood volume and stabilizes blood pressure during volume depletion or shock. The ability of colloids to remain within this compartment is essential for their effectiveness in therapeutic applications.

In contrast, while interstitial fluid, extracellular fluid, and cytoplasmic regions have their own roles, they do not retain colloids to the same extent as the intravascular compartment. For instance, colloids do not easily cross the capillary walls into the interstitial space due to their size, which is why they are effective for volume expansion when administered directly into the bloodstream.