Which among the following is NOT an example of a colloid?

• A. Plasma
• B. Hetastarch
• C. Blood products
• D. Normal saline

Answer: (D)

The key idea is the difference between colloids and crystalloids in IV fluids. Colloids contain large molecules that remain mainly in the intravascular space, creating higher oncotic pressure and drawing fluid into vessels. Crystalloids are small molecules that readily diffuse across capillary walls, so they quickly distribute into the interstitial space.

Plasma and blood products include proteins and other large molecules, so they behave as colloids and help expand the intravascular volume. Hetastarch is a synthetic polymer also classified as a colloid for the same reason. Normal saline, on the other hand, is a crystalloid consisting of small solutes (sodium and chloride ions); it diffuses out of the vasculature into the interstitial space and does not raise oncotic pressure, so it is not a colloid.

So the liquid that is not a colloid is normal saline.