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It is an important part in the story of blood 
when the blood flows out, the platelets release an enzyme called thrombokinase.
thrombokinase acts on another substance present in the blood called                pro-thrombin converting in to thrombin .
thrombin acts on another substance called fibrin,that is present in dissolved state converting it into insoluble fibrin.
the blood cells entangle in the fibrin fibers forming the clot.
the fibrin fibers are attached to the edges of the wound and pull them together.
 this yellowish coloured fluid portion after formation of the clot is called 'serum'