Coagulation defines the transformation of a substance from a liquid state to a more or less solid state. In practice, this term is used primarily to describe the blood and corresponds to the set of phenomena that stops bleeding. The clotting process begins with the appearance of a lesion in a blood vessel. First, platelets gather to block the blood from the breach. Then, fibrin fibers composing the blood, strengthen the clot. Many factors and molecules are involved in this complex set of reactions and several disorders can be detected from trouble coagulating due to these different factors.