Typical profiles of thrombin activity (FIIa) and clot density. (A-B) Thrombin concentration inside of the clot at the indicated time points (A) without added FXIa or (B) with 0.12 or 0.48 pM of FXIa. (C-D) Clot density profiles at the indicated time points (C) without added FXIa or (D) with 0.12 or 0.48 pM of FXIa. Profiles are shown for the experiments presented in Figure 1 (representative of 2--4 repeats).