Role of the endothelial surface coat in microvessel blood flow

The endothelial lining of blood vessels presents a large surface area for exchange of materials between blood and tissues and is critically involved in various phenomena providing homeostasis of tissue supply and function. The luminal surface of the endothelium is lined with a glycocalyx of membrane-bound macromolecules which according to electron microscopy is up to 100 nm thick. Intravital microscopy indicates an endothelial surface layer (ESL) that, due to incorporation of plasma proteins, is much thicker (0.5-1.5 mum), interferes with the free flow of plasma and causes exclusion of red blood cells from a considerable fraction of capillary cross section. The existence of this ESL, its composition, and its biophysical properties are functionally relevant for the physiological mechanisms of acute and chronic flow regulation.