Lungs, muscle, and other organs contain microvessels which form a continuous endothelial barrier. Fluid and small solutes cross this barrier through interendothelial junctions, but the larger proteins can only permeate by passing through either larger damaged junctions or large fused plasmalemmal vesicles. Fenestrated endothelial barriers are located in organs in which large amounts of transcapillary volume exchange occur, such as the gastrointestinal tract and renal capillaries. Continuous and fenestrated capillaries are very permeable to small solutes and water, but are relatively impermeable to plasma proteins since they contain numerous small pores of radius 4 to 5 nm and only a very few large pores of radius 20 nm. However, all plasma proteins leak into the interstitium in organs, except at the blood-brain barrier which is impermeable to all solutes, even small solutes which must cross by an active transport process. Capillaries with discontinuous barriers are found in liver sinusoids and have very large gaps in their walls (radius, 100-1000 nm) which allow even the largest plasma proteins to pass easily into the interstitial spaces.
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