In recent years, we have witnessed a rapid expansion of our knowledge regarding the role of the endothelium in the control of vascular tone (and organ perfusion) in health and disease. Physiology, pharmacology, and molecular biology have uncovered a wealth of information on structure and function of this heretofore largely neglected "organ." Clinical medicine is now called upon to define the clinical significance of these observa tions that imply the mechanisms of blood coagulation, e.g., the interaction of throm bocytes with the endothelium, vasomotor control, and specifically, the...