Our vascular system is constantly subjected to forces generated by blood flow. To prevent hemorrhage at injury sites, our body needs to sense injuries and initiate blood clotting to stop bleeding. von Willebrand factor (VWF) is an important mechanosensory molecule, which constantly scans our bodies for injuries and plays a critical role in platelet adhesion and aggregation during blood clotting. By combining single-molecule fluorescence imaging and microfluidics systems, we simultaneously monitored how the hydrodynamic forces regulate the inter- and intro-molecular interactions in single VWF molecules and their binding to platelets. Live imaging in this system reveals a novel mechanism that enables VWF to be locally activated by hydrodynamic force in hemorrhage and rapidly deactivated downstream, with implications for therapy development in bleeding disorders and thrombosis.