NOS3 and cardiovascular disorder: Our study, using quantitative measurement of microvessel permeability in combination with fluorescence and confocal microscopy, identified a novel mechanism by which H2O2, a commonly known reactive oxygen species essential for the pathogeneses of cardiovascular diseases, interplays with superoxide, eNOS‐derived NO, and ONOO−, leading to EC Ca2+ overload, vascular cell damage, and barrier dysfunction in intact vessels.