Blood Coagulation

Abstract The blood coagulation system acts in concert with the platelets to seal damaged blood vessels by the formation of a clot that consists of aggregated platelets interwoven with fibrin. Coagulation is initiated by the subendothelial membrane protein tissue factor and proceeds through a highly regulated proteolytic cascade to the production of thrombin and the fibrin clot. Although stoppage of blood leakage is the primary purpose of the coagulation system, many of the direct products of coagulation have additional functions that involve communicating with surrounding cells and crosstalk with other biochemical systems (i.e. inflammation and immunity). These secondary roles of the coagulation system products are vital for regeneration of tissue normalcy and overall homeostasis. Dysregulation of any of these coagulant system functions can lead to various clinical pathologies. Key Concepts The initiator of coagulation, tissue factor, surrounds the vasculature in a protective haemostatic envelope. The cascade design and feedback reactions in blood coagulation serve to rapidly amplify and dampen the response to create a burst of activity rather than a prolonged response. Coagulation throw‐off products serve to signal the surrounding tissue to protect against pathogen invasion as well as initiate wound healing. Platelets and platelet‐derived polyphosphate serve to propagate coagulation via the intrinsic pathway and help localise coagulation to the site of damage. Dysregulation of the coagulation system upsets the delicate haemostatic balance and can result in numerous clinical pathologies.