Blood coagulation is the transformation of blood as a liquid into a solid gel-like blood "clot". A clot is formed when platelets stick to collagen exposed by a disrupted blood vessel lining. Other platelets then bind to the initial platelets to form what is called a platelet plug. The platelet plug is limited to the disrupted area and does not spread to the normal unaffected tissue. Clotting occurs on top of the platelet plug when Fibrinogen (a large soluble Protein) is converted into Fibrin monomers (an insoluble thread-like molecule) by thrombin. Fibrin monomers are then crosslinked into Fibrin polymers in the presence of factor XIIIa and Ca2+.