Which statement best describes net filtration pressure in capillaries?

• A. Net filtration pressure equals capillary hydrostatic pressure minus blood colloid osmotic pressure
• B. Net filtration pressure equals oncotic pressure minus hydrostatic pressure
• C. Net filtration pressure equals total capillary pressure plus oncotic pressure
• D. Net filtration pressure is unrelated to hydrostatic pressure

Answer: (A)

Fluid movement across capillary walls is driven by a tug-of-war between forces that push fluid outward and forces that pull fluid inward. The main outward push is capillary hydrostatic pressure, while the main inward pull is blood (capillary) colloid osmotic pressure due to plasma proteins. Net filtration pressure captures the balance of these forces and is defined as capillary hydrostatic pressure minus blood colloid osmotic pressure. If this difference is positive, filtration (fluid leaving the capillary) dominates; if negative, reabsorption (fluid entering the capillary) occurs. In the usual simplified view, interstitial pressures are not included, so the formula focuses on these two forces.