Starling forces that oppose filtration after tissue oncotic pressure is increased

We tested the hypothesis that the effective oncotic force that opposes flui d filtration across the microvessel wall is the local oncotic pressure diff erence across the endothelial surface glycocalyx and not the global differe nce between the plasma and tissue. In single frog mesenteric microvessels p erfused and superfused with solutions containing 50 mg/ml albumin, the effe ctive oncotic pressure exerted across the microvessel wall was not signific antly different from that measured when the perfusate alone contained album in at 50 mg/ml. Measurements were made during transient and steady-state fi ltration at capillary pressures between 10 and 35 cmH(2)O. A cellular-level model of coupled water and solute flows in the interendothelial cleft show ed water flux through small breaks in the junctional strand limited back di ffusion of albumin into the protected space on the tissue side of the glyco calyx. Thus oncotic forces opposing filtration are larger than those estima ted from blood-to-tissue protein concentration differences, and transcapill ary fluid flux is smaller than estimated from global differences in oncotic and hydrostatic pressures.