Which process is used for reabsorption across capillary walls?

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

Answer: (D)

Water reabsorption across capillary walls is driven by osmosis. Water moves across the capillary endothelium toward the side with higher solute concentration, which in capillaries is the plasma side due to plasma proteins (albumin) creating oncotic pressure. This osmotic pull draws water back from the interstitial space into the blood, balancing the earlier fluid loss driven by hydrostatic forces.

In this system, hydrostatic pressure pushes water and small solutes out of capillaries at the arterial end (filtration), while the opposing oncotic/osmotic pressure pulls water back in at the venous end (reabsorption). Active transport is not needed for this water movement, and diffusion refers to solutes moving down their concentration gradients. Filtration describes the outward bulk flow driven by hydrostatic pressure, not the inward reabsorption.