VWF and hemophilia A: Specifically, we show that: (a) under physiologic flow conditions, the anti-platelet agent eptifibatide predominantly affects hemostatic plug formation via attenuation of platelet aggregate density and clot contraction ; (b) inhibition of vWF prolongs the in vitro bleeding time in a shear-dependent manner; (c) phosphatidylserine (PSer) is expressed primarily on the surface of endothelial cells at the site of vascular injury; and (d) blood from hemophilia A patients features abnormal fibrin architecture and significantly increased bleeding times.