When you cut your finger, it starts to bleed, but after some time, the bleeding stops. This is because your blood contains special substances that male the blood clot.
To begin, with your blood contains tiny components called platelets. Platelets become stimulated when they encounter a damaged blood vessel, and flock to the site. The platelets clump together and form a plug, which reduces bleeding. blood also contains dissolved proteins, called clotting factors, which promote blood clots. They trigger in a rapid chemical chain reaction whose end product is fibrin, the main protein forming blood clots.
Once formed, the blood clot must be prevented from growing, and spreading through the body, where it could cause damage. Numerous anti clotting proteins exist in a natural balance with clotting factors. The anti-clotting enzymes prevent the blood clot from extending farther than it should go. Then, as the damaged tissue heals, the body slowly degrades the blood clot, and reabsorbs it.