Flow arrest triggers clot contraction. A thrombus formed in the absence of thrombin and presence of fluorescent 50 nm beads was rinsed with Ca²⁺ buffer for 4.5 min before the cessation of flow caused a rapid contraction. The outline of a pre-retracted thrombus (t'=−1 min) shows the inward retraction of the thrombus following flow stoppage (t'=0 to 2 min) (A). Contraction rate of the upstream and downstream sections of the thrombus were measured before and after flow arrest (n=3 donors) (B). Trajectories of the 50 nm beads represent the contractile response of the thrombus at upstream (red, n=6), middle (blue, n=3), and downstream (green, n=6) locations (C). Stopping the flow caused a significant increase in contraction rate in the Y and Z directions. To quantify these rates, the times before (0–2 min) and after flow cessation (0–1 min, 1–2 min) were monitored for bead velocity in the three sections of the thrombus (D). Downstream contraction rate in Y direction is shown as absolute value for contraction toward the middle region. *, P<0.01; error bars indicate mean ± SD.