Coagulation – also known as clotting – is a critical part of hemostasis, which is a complex process in which various signaling pathways and factors work together to seal ruptured blood vessels and stop further bleeding.

Coagulation Signaling

Hemostasis is the body's process to prevent excessive bleeding by forming blood clots and maintaining the integrity of blood vessels. When a blood vessel gets damaged, a vascular spasm causes the vessel walls to contract and reduce blood flow in the area. Platelets begin to accumulate in the area and clump together to form a stable plug. Then the coagulation system and a cascade of reactions convert soluble fibrinogen into insoluble fibrin, forming a mesh-like network that stabilizes the clot. This process stops bleeding and helps restore normal vascular function.

The coagulation system includes two major signaling cascades: the intrinsic prothrombin activation pathway and the extrinsic prothrombin activation pathway. The intrinsic pathway, also known as the contact activation pathway, is initiated when blood comes into contact with negatively charged surfaces, such as damaged endothelial cells or collagen in the blood vessel wall. The intrinsic pathway requires numerous steps and interactions with clotting factors in the blood and typically takes a few minutes to complete.

The extrinsic pathway, which is also called the tissue factor pathway, is much faster and is completed in a few seconds. The extrinsic pathway provides a rapid response to traumatic injuries because it involves a direct activation by tissue factor, which is present outside of blood vessels and is readily accessible when tissue is damaged.

The GP6 (glycoprotein VI) signaling pathway is critical to the primary stage of hemostasis, as it is responsible for platelet activation and aggregation. Platelets are small cell fragments that circulate in the blood, and upon injury or damage to blood vessels, they are recruited to the site to help seal the wound.

The pathway becomes activated when the GP6 receptor on the surface of platelets encounters and binds to collagen, a protein found in the extracellular matrix of damaged tissues. This sets off a cascade of events that results in platelet shape change, adhesion to damaged blood vessels, aggregation with other platelets and the release of granules containing clotting factors and other substances necessary for blood clot formation

Both coagulation pathways converge upon the activation of factor X, and the so-called common pathway continues, resulting in the conversion of prothrombin to thrombin. Thrombin is an enzyme that is crucial to coagulation, because it cleaves the soluble plasma protein fibrinogen into fibrin, which stabilizes the blood clot by forming a mesh-like network.

Without proper coagulation, even minor injuries could result in potentially life-threatening bleeding. Therefore, the coagulation system must maintain a delicate balance between clot formation and prevention of excessive clotting. Dysregulation or imbalances in the coagulation system can lead to a variety of medical conditions. For example, thrombosis occurs when blood clots form inappropriately within blood vessels. This can result in serious medical conditions, such as deep vein thrombosis (DVT), pulmonary embolism or stroke.

Bleeding disorders such as hemophilia, in which blood clotting is impaired, can stem from deficiencies or abnormalities in certain factors of the coagulation system, most commonly factor VIII (hemophilia A) or factor IX (hemophilia B). Individuals with hemophilia experience prolonged bleeding, even from minor injuries, and the condition can lead to spontaneous bleeding into joints, muscles and other tissues, causing pain, swelling and potentially permanent damage. If left untreated or inadequately managed, life-threatening bleeding can occur.

Understanding the complexities of the coagulation system and its pathways is important for researchers and clinicians. It allows for the development of diagnostic tests to assess the functionality of the system and to identify abnormalities. It also supports the development of therapeutics, such as anticoagulants for preventing and treating thrombotic disorders, as well as clotting factors and platelet transfusions to manage bleeding disorders.



OpenStax College, Anatomy & Physiology. OpenStax College. 19 June 2013., accessed July 2023.

Brummel-Ziedins K, Mann KG. Chapter 128 - Molecular Basis of Blood Coagulation. In: Hoffman R, et al, eds. Hematology (Sixth Edition), Elsevier, 2013.

Palta S, Saroa R, Palta A. Overview of the coagulation system. Indian J Anaesth. 2014 Sep;58(5):515-23. doi: 10.4103/0019-5049.144643.