List the three important processes that move materials across typical capillary walls.

• A. Diffusion, filtration, and reabsorption
• B. Osmosis, diffusion, and filtration
• C. Diffusion, filtration, and osmosis
• D. Filtration, osmosis, and active transport

Answer: (A)

Across typical capillary walls, three main processes move materials: diffusion, filtration, and osmosis. Diffusion allows solutes to spread along concentration gradients through the endothelial lining or pores, enabling gases like O2 and CO2 and small ions to pass. Filtration is bulk flow driven by differences in hydrostatic pressure, pushing water and small solutes out of the capillary into the interstitial space, especially at the arterial end. Osmosis moves water across the capillary wall in response to solute (osmotic) gradients, helping balance concentrations. Reabsorption isn’t a separate transport mechanism across the capillary wall; it describes the net inward movement of fluid back into the capillary driven by oncotic and other Starling forces, and is a consequence of the balance between filtration and osmotic pressures rather than a distinct crossing process.