Computer simulation for FXIa activation and inhibition in LPS volunteers. A mathematical model was constructed to explain the course of FXIa concentrations as measured in the ECA and ELISAs. The inset shows a scheme of the assumed transfer of substances, in which the formation of FXIa-FXIa inhibitor complexes occurs outside the fluid phase plasma compartment. The graph shows the calculated time course for the concentrations of noncomplexed FXIa (— - —) and FXIa-FXIa inhibitor complexes (—) in the plasma compartment assuming inhibition of FXIa outside the plasma compartment, a slow dissociation of bound FXIa-FXIa inhibitor complexes into the plasma compartment, and a plasma half-life of FXIa-FXIa inhibitor complexes of 100 minutes. The left y-axis represents the absolute increase in FXIa and FXIa-FXIa inhibitor complexes in pmol/L, the right y-axis represents the formation of endogenous FXIa (----) in pmol/minute. The measured FXIa in the ECA (▴) and FXIa-FXIa inhibitor complexes (•) are given for comparison.