Net Filtration Pressure across a capillary is calculated as:

• A. (capillary hydrostatic pressure - interstitial hydrostatic pressure) - (capillary oncotic pressure - interstitial oncotic pressure).
• B. (interstitial hydrostatic pressure - capillary hydrostatic pressure) - (capillary oncotic pressure - interstitial oncotic pressure).
• C. (capillary hydrostatic pressure - interstitial hydrostatic pressure) - (interstitial oncotic pressure - capillary oncotic pressure).
• D. (capillary hydrostatic pressure - interstitial hydrostatic pressure) - (capillary oncotic pressure - interstitial oncotic pressure)

Answer: (D)

Net filtration pressure across a capillary comes from the balance of Starling forces: hydrostatic pressure pushes fluid out of the capillary, while oncotic (colloid osmotic) pressures pull fluid in. The outward force is the capillary hydrostatic pressure minus the interstitial hydrostatic pressure, and the inward force is the capillary oncotic pressure minus the interstitial oncotic pressure. Put together, the net filtration pressure is

NFP = (Pc − Pi) − (πc − πi)

This formulation captures that fluid moves out when Pc is high or Pi is low, and moves in when πc is high or πi is high. The given correct expression uses those exact differences: capillary minus interstitial hydrostatic, minus the difference between capillary and interstitial oncotic pressures.