Abstract The endothelial lining of blood vessels presents a large surface area for 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 endothe- lium is lined with a glycocalyx, a layer of membrane- bound macromolecules which has been determined by electron microscopy to be several tens 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 μm to over 1 μm, that restricts the flow of plasma and can exclude red blood cells and some macromolecular sol- utes. The evidence for the existence of the ESL, hypoth- eses about its composition and biophysical properties, its relevance to physiological processes, and its possible clinical implications are considered in this review. Key words Biophysical properties · Endothelial surface layer · Fluid and solute exchange Evidence for the existence of the endothelial surface layer Introduction By extrapolating from data of Mall [63] on the anatomi- cal dimensions of the vascular system in the canine in- testine [48, 92], the total area of the blood/endothelium interface in humans can be estimated to be about 350 m 2 . For an estimated average endothelial thickness of about 0.3 μm, this corresponds to a total endothelial mass of only 2245110 g. Nevertheless, the endothelium is by no means just a passive membrane or barrier between blood and tissues. On the contrary, it is actively involved in many functions, such as the control and regulation of vascular tone, fluid and solute exchange, haemostasis and coagulation and inflammatory responses. Most of these functions depend critically on phenomena occur- ring at the endothelial surface in the vascular lumen. The physical and chemical nature of the surface presented by endothelial cells to the flowing blood can therefore be assumed to influence many of these aspects of vascular function. The concept that vessel walls are lined with an extra- cellular layer of membrane-bound substances dates back more than 50 years. Initially, studies of endothelial per- meability initiated an interest in the structure and com- position of the material on the endothelial surface and in the inter-endothelial clefts. Danielli [28] and Chambers and Zweifach [18] introduced the concept of a thin non- cellular layer on the endothelial surface (endocapillary layer) which was thought to include adsorbed plasma proteins. Direct observation of this layer proved to be technically impossible. However, during perfusion of the frog mesentery with a Ringer’s solution containing Evans blue, which has a high affinity for albumin, Chambers and Zweifach [18] observed “thin strands and sheets of a faintly colored blue, translucent material … sloughing off the inner surface of the capillary.” About 20 years later, Copley [21, 22] studied the endotheli- um–plasma interface and developed a concept (Fig. 1) in which the endothelial surface is covered by a thin molec- ular layer and an immobile sheet of plasma. The exis- tence of the latter was inferred from intravital micro- scopic experiments in the hamster cheek pouch [23]. Af- ter intravenous injection of a dye (pontamine sky blue) an unstained plasmatic zone adjacent to the endothelial surface was observed. In 1966, Luft [62] used ruthenium red staining for an electron microscopic study of the endothelial surface. A.R. Pries · P. Gaehtgens Dept. of Physiology, Freie Universität Berlin, Arnimallee 22, 14195 Berlin, Germany A.R. Pries Deutsches Herzzentrum Berlin, Augustenburger Platz 1, 13353 Berlin, Germany T.W. Secomb Dept. of Physiology, University of Arizona, Tucson, AZ 85724, USA Pflügers Arch – Eur J Physiol (2000) 440:653–666 DOI 10.1007/s004240000307 INVITED REVIEW A.R. Pries · T.W. Secomb · P. Gaehtgens The endothelial surface layer Received: 8 December 1999 / Received after revision: 3 March 2000Accepted: 6 March 2000 / Published online: 21 April 2000 © Springer-Verlag 2000