Purpose of review: Vascular permeability is traditionally explained by Starling's principle, describing two opposing forces across the endothelial cell line to maintain compartments in balance. Several contradictions to this principle have recently questioned its validity.
Recent findings: Hydraulic conductivity is kept low by a properly working endothelial surface layer, created by binding and intercalating plasma constituents with the structural elements of an endothelial glycocalyx. Limiting fluid filtration is not closely related to the interstitial protein concentration. Rather, the oncotic pressure difference pertinent to fluid homeostasis is built up between the intravascular space and a small protein-free zone beneath the protein-loaded endothelial glycocalyx. This crucial structure, and therefore the resistance of the barrier against outflow of large molecules, is endangered by ischaemia, inflammation and intravascular hypervolaemia. An intact endothelial surface layer retains iso-oncotic preparations of large molecules infused to compensate for acute bleeding. Crystalloids cannot be held back sufficiently, even if preload is warranted.
Summary: Starling's principle requires an adaptation to recognize that there is no inward-directed oncotic pressure gradient across the whole anatomical vessel wall. The carrier of vascular barrier competence is the intact endothelial surface layer which might be protected by avoiding intravascular hypervolaemia and limiting inflammation.