In blood coagulation, the rate-limiting step is:

Activation of factor X or formation of prothrombin activator (Xa) is the rate-limiting step in the clotting cascade. Extrinsic pathway for coagulation: Tissue trauma --> release of tissue thromboplastin --> eventual conversion of X to Xa. Intrinsic pathway for coagulation: Trauma to blood, or blood (factor XII) coming in contact with subendothelial collagen --> other clotting factors activated in the subsequent cascade --> eventual conversion of X to Xa.