The body uses the complex system of hemostasis to form blood clots, during which the blood changes from a liquid to a more gel-like consistency. When a vessel receives damage, the blood moves outside of the vein and the body begins an innate process to stop the bleeding. First, the blood vessels constrict to reduce blood loss. Next, platelets begin to stick together to seal the break in the vessel by forming a platelet plug. Finally, clotting factors -- inactive proteins found in the blood plasma -- activate and form a fibrin mesh that traps some of the red and white blood cells. This results in a hardening of the platelet plug, which becomes a clot.
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