The Starling forces that oppose filtration in the presence of tissue loading

The net fluid flow across the capillary walls and the formation of lymph are governed by the imbalance of the hydrostatic pressure and protein osmotic pressure difference across the vessel wall. Contrary to the current interpretation of the Starling forces, we propose that the effective oncotic force that opposes fluid filtration is the local oncotic pressure difference across the endothelial surface glycocalyx and not the global difference in oncotic pressure between the plasma and tissue as nearly universally assumed until now. Here we show, this local difference can differ greatly from the global oncotic pressure difference due to the presence of the junction strands which greatly inhibits back diffusion from the tissue space. This leads to a dramatic change in the fluid flux and a large reduction in current estimates of the filtration flow based on tissue oncotic pressure.