Which statement best describes capillary exchange in terms of mechanisms?

• A. Diffusion only moves materials across capillary walls
• B. Capillary exchange occurs via diffusion, filtration, and reabsorption
• C. Osmosis moves all solutes across capillary walls
• D. Active transport moves solutes across capillary walls

Answer: (B)

Capillary exchange relies on multiple mechanisms: diffusion for solute movement, and bulk flow (filtration and reabsorption) for fluid movement. Diffusion lets substances move down their concentration gradients across the capillary wall. Filtration pushes fluid out of capillaries on the arterial side due to capillary hydrostatic pressure, while reabsorption pulls fluid back in on the venous side thanks to plasma oncotic (protein) pressure. Osmosis describes water movement driven by osmotic gradients, but it’s not about moving all solutes, and it isn’t the sole mechanism of exchange. Active transport can move specific substances, but the main capillary exchange processes are diffusion plus filtration and reabsorption. This combination is why the statement describing diffusion, filtration, and reabsorption best captures how capillary exchange occurs.