Which process is commonly used for capillary reabsorption, describing solvent movement across a membrane?

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

Answer: (D)

Osmosis is at work here—the movement of water across a semipermeable membrane driven by an osmotic gradient. In capillaries, plasma proteins remain inside the blood, creating higher oncotic (osmotic) pressure inside the capillary than in the surrounding interstitial fluid. This gradient pulls water from the interstitial space back into the blood, producing capillary reabsorption.

Filtration, by contrast, is the bulk movement of water and solutes driven mainly by hydrostatic pressure differences across the capillary wall, and diffusion involves solute movement, not solvent, across membranes. Active transport requires energy and moves substances against gradients, not the passive solvent flow described by osmosis.