Blood clots form to help prevent leaking when you break the surface of the skin — for example, when you cut your finger chopping carrots or on a piece of broken glass. Your body goes through a certain series of events to make sure that you do not bleed to death.
First, after you shriek in pain or mutter a few choice words, the injured blood vessel constricts. The constriction reduces blood flow to the injured blood vessel, which helps to limit blood loss. Tourniquets help to squeeze off blood flow in much the same way when major blood vessels are injured.
With the injured blood vessel constricted, the platelets present in the blood that is passing through that vessel start to stick to the collagen fibers that are part of the blood vessel wall. Eventually, a platelet plug forms, and it fills small tears in the blood vessel.
Once the platelet plug is formed, a chain of events is initiated to form a clot. The reactions that occur are catalyzed by enzymes called clotting factors. There are 12 clotting factors, and the mechanism that occurs is very complex.
Here are just a few highlights of what happens during the clotting process:
After a platelet plug forms, the coagulation phase begins, which involves a cascade of enzyme activations that lead to the conversion of prothrombin to thrombin. Calcium is required for this reaction to occur.
Thrombin itself acts as an enzyme and causes fibrinogen — one of the two major plasma proteins — to form long fibrin threads.
Fibrin threads entwine the platelet plug forming a mesh-like framework for a clot.
The framework traps red blood cells that flow toward it, forming a clot. Because red blood cells are tangled in the meshwork, clots appear to be red. As the red blood cells trapped on the outside dry out, the color turns a brownish red, and a scab forms.