Osmotic pressure is created by the presence in a fluid of small diffusible molecules that easily move through the capillary membrane.

• A. True
• B. False
• C. Not enough information
• D. Depends on temperature

Answer: (A)

Osmotic pressure arises from solute particles that cannot cross the capillary membrane easily. Water will move across the membrane to balance solute concentrations, creating pressure that opposes that flow. If the solutes are small and freely diffusible, they equalize across the membrane and do not sustain an osmotic pressure difference. In blood vessels, the key contributor to osmotic (oncotic) pressure is large, non-diffusible plasma proteins like albumin that stay in the intravascular space and pull water back into capillaries. So the statement—that osmotic pressure is created by small diffusible molecules that easily move through the membrane—is not correct.