Activation of coagulation by platelet polyphosphate. Platelet dense granules contain polyphosphate polymers (shown as beads) and high concentrations of calcium. Upon platelet activation, polyphosphate polymers of varying sizes are released. Shorter polymers are soluble and promote coagulation by serving as cofactors for thrombin-mediated activation of FXI and FV, whereas longer polymers deposit as nanoparticles on the platelet surface, where they may initiate coagulation by activating FXII. HK, high molecular weight kininogen; PK, prekallikrein.