After initiation, each step in the intrinsic pathway is catalysed by the product of the preceding step:
factor XII, also termed Hageman factor, is converted to XIIa on contact with negatively-charged surfaces e.g. that of subendothelial collagen. For activation to its enzymatic form, it requires high molecular weight kininogen and prekallikrein.
factor XIIa converts factor XI to XIa
factor XIa converts factor IX converted to IXa
factor IXa converts factor X to Xa; to do this it requires:
a negatively charged surface; in vivo, this is the surface of platelets so localizing the cascade
factor Xa then activates the common pathway of coagulation