The movement of fluid from tissues toward a higher concentration of sodium in the blood is an example of which process?

• A. Diffusion
• B. Filtration
• C. Active transport
• D. Osmosis

Answer: (D)

This question centers on osmosis—the movement of water across a semipermeable membrane toward a higher solute concentration. Here, the blood has more dissolved particles (such as sodium) than the surrounding tissue fluid, so water moves from the tissue fluid into the blood to balance solute levels. The key is that water is moving to the side with higher solute concentration, not solutes moving on their own. Diffusion would involve solutes moving down their own concentration gradient, filtration is driven by pressure differences, and active transport requires energy to move substances against gradients. So the described movement is osmosis.