A formula used to calculate the net flow of fluid over a semipermeable (capillary) membrane: J = K [(Pc-Pi) - σ(πc - πi)] (J = net fluid flow, K = capillary fluid permeability constant, Pc = capillary pressure, Pi = interstitial pressure, σ = capillary protein permeability constant, πc = capillary oncotic pressure, and πi = interstitial oncotic pressure)