F2 and respiratory failure: From a biological standpoint, it appears that prey immobilization and death induced by Deinagkistrodon acutus venom may occur by their different and highly effective mechanisms: intravascular thrombosis provoked by the prothrombin activator, thrombin and serine proteinase, blood coagulation disorders induced by the antithrombin, metalloproteinases and serine protease, and paralysis leading to respiratory failure induced by the PLA2.