The microcirculation consists of a highly dynamic system of vessels that not only subserves the local metabolic needs of individual organs, but also functions as a site for integration of a variety of physiological processes, including regulation of arterial pressure (in arterioles), plasma volume (capillaries), and inflammation (venules). This integrative function of the microvasculature can be largely attributed to the responses of a single cellular component of the vessel wall, ie, the endothelial lining. There is growing recognition that dysfunctional endothelial cells contribute to the pathogenesis of a number of cardiovascular diseases, including atherosclerosis, hypertension, and shock. This paper reviews: (i) the morphology and ultrastructure of the microcirculation, and (ii) the functions and responses of three key elements of the microcirculation, ie, arterioles, capillaries, and venules, in health and disease...