when there is an injury, the platelets in the blood comes in contact with air they decompose to form thromboplastin, thromboplastin combine with inactive prothrombin, formed by the combination of Vitamin K and Calcium ions, to form active thrombin, thrombin combines with the fibrinogen in the blodd plasma to form Fibrin, this Fibrin entangle with blood corpuscles making the clot form
Therefore your answer would be Fibrin.
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