Ace the 2026 A&P Blood Vessels Challenge – Pump Up Your Knowledge!

Both diffusion and bulk flow contribute to fluid exchange. Diffusion moves solutes and gases across membranes by their concentration gradients, which is efficient for small molecules and can occur over short distances. Bulk flow moves the entire solvent with dissolved substances as a result of pressure differences—hydrostatic pressure pushing out and oncotic pressure pulling in—allowing rapid movement of fluid through capillary walls.

In practice, capillary exchange relies on both: small solutes such as O2, CO2, and glucose diffuse across the endothelium, while water moves with the solvent due to Starling forces, producing filtration at the arterial end and reabsorption toward the venous end. The statement is true because both mechanisms operate together to achieve fluid exchange; their relative contribution varies by tissue, distance, and gradient, so they are not always equal in magnitude but are both essential.