Final answer:
The propensity of a contrast agent to cause fluid to move from the extravascular space to the intravascular space is called tonicity, which is how an extracellular solution can change the volume of a cell by affecting osmosis.
Step-by-step explanation:
The propensity of a contrast agent to cause fluid to move from the extravascular space to the intravascular space is called Tonicity. Tonicity describes how an extracellular solution can change the volume of a cell by affecting osmosis. It often directly correlates with the osmolarity of the solution, which describes the total solute concentration of the solution. In a situation where solutions of different osmolarities are separated by a membrane permeable to water, water will move from the side with lower osmolarity (and more water) to the side with higher osmolarity (and less water), following its concentration gradient.