How does our coagulation system work?
Our blood is prevented from flowing endlessly by three main mechanisms:
- Our blood vessels: Blood vessels aren’t rigid pipes and their walls are formed of elastic fibers with muscles. If a blood vessel is injured, it will reflexively contract, causing a decrease in blood flow, minimizing blood loss.
- Platelets: Platelets are fragments of larger cells in the bone marrow called megakaryocytes. Platelets have different functions and lay the groundwork for forming the blood clot. They are considered the first responders when a blood vessel injury occurs.
- Coagulation factors: Coagulation factors are proteins present in an inactive form. They are activated in a cascade-like manner by two pathways, the extrinsic and intrinsic ones. Coagulation factors are mostly produced by the liver and are the main site of action of blood thinners like Warfarin and Heparin.
When the blood vessel is injured, its inner lining becomes rough, stimulating platelets to adhere to it. Platelets then signal each other to aggregate on the defect occluding it. Platelets also start the coagulation cascade through the activation of factor XII (12) by the release of specific polymers. The cascade proceeds in a domino-like fashion and ends by the formation of fibrin threads. These threads eventually trap red blood cells, white blood cells, and platelets forming the blood clot.
For coagulation to occur, a disturbance must happen either in the intactness of blood vessels lining, the activation of coagulation factors, or the regular, seamless flow of blood. This is called Virchow’s triad.