Which force drives filtration across capillary walls?

• A. Capillary hydrostatic pressure
• B. Plasma oncotic pressure
• C. Interstitial hydrostatic pressure
• D. Lymphatic pressure

Answer: (A)

Filtration across capillary walls is driven by capillary hydrostatic pressure, the outward force exerted by blood against the capillary walls. This pressure pushes water and small solutes from the blood into the surrounding interstitial space, especially at the arterial end of the capillary. The main opposing forces are plasma oncotic pressure, which pulls fluid back into the capillaries due to plasma proteins like albumin, and interstitial hydrostatic pressure, which tends to oppose outward flow. Lymphatic drainage helps prevent excess accumulation but is not the primary driver of filtration. Because the outward push from capillary hydrostatic pressure is the principal force moving fluid out of the capillaries, it best explains filtration.