The endothelial surface layer

The endothelial lining of blood vessels presents a large surface area fur exchange of materials between blood and tissues, and is critically involved in many other processes such as regulation of blood flow, inflammatory responses and blood coagulation. It has long been known that the luminal surface of the endothelium is lined with a glycocalyx, a layer of membrane-bound macromolecules which has been determined by electron microscopy to be several tons of nanometers thick. However, investigations in vivo have indicated the presence of a much thicker endothelial surface layer (ESL), with an estimated thickness ranging from 0.5 mum to over 1 mum, that restricts the flow of plasma and can exclude red blued cells and some macromolecular solutes. The evidence for the existence of the ESL, hypotheses about its composition and biophysical properties, its relevance to physiological processes, and its possible clinical implications are considered in this review.