Activities and functions of polyP in the coagulation system. Two different pools of polyP that differ in their biophysical properties and procoagulant mechanisms, respectively, operate in the coagulation cascade. Insoluble long-chain polyP and polymers condensed in microparticles initiate FXII contact activation on cell surfaces. Although mechanisms driven by insoluble polyP microparticles have been analyzed in vivo, functions of soluble/short-chain polymers are based on in vitro studies.