The major clot-forming substrate in the blood plasma of vertebrates. Though fibrinogen represents a small fraction of plasma proteins (normal human plasma has a fibrinogen content of 2–4 mg/ml of a total of 70 mg protein/ml), its conversion to fibrin causes a dramatic gelation which blocks the flow of blood. Upon injury, sufficient amounts of the clotting enzyme, thrombin, are generated in about 5 min clotting time to produce a gel.