Vascular endothelium is a highly differentiated cellular monolayer with the organization of a simple squamous epithelium. It lines the entire cardiovascular system and thus constitutes a quasi-ubiquitous presence in organs and tissues throughout the body. One of the most significant aspects of the vascular endothelium is its diversity of phenotype and function, which endows it with the ability to regulate the exchange of myriad substances between blood plasma and interstitial fluid in all tissues of the body. The ability of the endothelium to regulate the transendothelial exchange or vascular permeability is critically important in the growth, maintenance, and survival of all tissues, as well as in the delivery of therapies to correct locations.

Within the circulation, exchanges occur across the endothelium at all levels of the vascular tree. However, two functional segments are recognized with respect to the role of the vessel wall in the transendothelial exchanges between blood and tissues served: the conduit vessels and the exchange vessels. The conduit vessels take blood from heart to tissues (large arteries down to the smaller muscular arterioles upstream of the precapillary sphincter) or from tissues to heart (small muscular veins collecting from venules and draining into large veins). In these vessels (with large sectional diameter but low aggregated exchange surface), the transendothelial exchanges are thought to have impact only on the vessel wall and not on the organs/tissues crossed by these vessels. The exchange vessels consist of capillaries and venules, and are thought to be involved in active exchanges between blood and tissues.