The body’s coagulation system functions to protect the pressurized circulatory system by both producing and preventing blood clots. This complex process is referred to as hemostasis. The coagulation system is comprised of three major components: endothelium, platelets and coagulation proteins.
Under normal conditions, these components work together to preserve the viscous nature of the blood circulating throughout the body. When an injury occurs, compromising the integrity of the pressurized circulatory system, these components interact to produce blood clots at the source of the damage. Compromised coagulation systems can result from disorders affecting the blood’s ability either to produce clotting, such as hemophelias and von Willebrand's disease, or to prevent unnecessary clotting, called thrombotic disorders.
The hemostatic process involves three actions that are regulated by the coagulation system simultaneously. Platelets bond to injured endothelial cells, forming a platelet plug. The coagulation process produces thrombin, which leads to the long-term stabilization of the platelet plug with fibrin strands, forming a clot. The expansion of the clot is controlled by anticoagulants produced in the endothelial cells.
Endothelial cells are one of the three primary components of the coagulation system. These cells are responsible for both triggering coagulation and limiting it to the site of the wound. Without these cells, blood would either fail to clot when needed or completely clot throughout the body’s blood vessels.
Located in the interior walls of blood vessels, endothelial cells perform their homeostatic function by producing and storing von Willebrand factor (vWF). This large protein functions as a glue to hold platelets to each other and to the cellular matrix around the wound site. Endothelial cells also synthesize a protein called tissue factor, which is involved in the production of thrombin, the chief agent in coagulation.
An anticoagulant function is also served by endothelial cells through their production of tissue factor pathway inhibitor (TFPI) and prostacyclin. TFPI inhibits the tissue factor complex, preventing formation of thrombin. Prostacyclin inhibits platelet aggregation. Together, these anticoagulant agents maintain the fluidity of the circulatory system and limit the scope of clotting.
Platelets are the second major part of the coagulation system. Platelets, also called thrombocytes, are tiny, non-nucleated, cytoplasmic fragments that are found flowing freely throughout the body’s blood vessels. Formed in the bone marrow, platelets contain various receptors that enable them to bond together and to damaged endothelial cells at the site of a wound. They are held together by von Willebrand factor, forming a platelet plug.
Coagulation proteins comprise the third major component of the coagulation system. These proteins lead to the formation of fibrin strands, which grow to form blood clots. There are 14 major coagulant proteins, including prothrombin, fibrinogen and tissue factor. Prothrombin and tissue factor combine in a complicated process to form thrombin, the primary agent in coagulation. Every aspect of hemostasis is involved in carefully managing it within the circulatory system.
