The vascular endothelium was initially viewed as a “semipermeable” barrier between circulating blood and the interstitium and organ parenchyma. It is now evident that the endothelium plays a much more complex and active physiological and pathophysiological role. For example, endothelial cells regulate vascular tone by the release of vasoactive mediators, such as nitric oxide and endothelin, and orchestrate the response to inflammatory stimuli by the expression of vascular adhesion molecules and increased “leakiness” to circulating cells and proteins.