Hemostasis is the process of blood clot formation at the site of vessel injury. When a blood vessel wall is disrupted, the hemostatic response must be quick, localized, and carefully regulated. Abnormal bleeding or thrombosis (ie, nonphysiologic blood clotting not required for hemostatic regulation) may occur when specific elements of these processes are missing or dysfunctional.
- The pathways of thrombin-mediated fibrin clot formation and plasmin-mediated clot lysis are linked and carefully regulated. When they work in coordinated harmony, a clot is laid down initially to stop bleeding, followed by eventual clot lysis and tissue remodeling.
- Abnormal bleeding can result from diminished thrombin generation (eg, due to factor VIII deficiency) or enhanced clot lysis (eg, due to alpha-2-antiplasmin deficiency). Conversely, excessive production of thrombin (eg, due to an inherited thrombophilia) can lead to thrombosis.
Two pathways of hemostasis
There are two main components of hemostasis. Primary hemostasis refers to platelet aggregation and platelet plug formation. Platelets are activated in a multifaceted process (see below), and as a result they adhere to the site of injury and to each other, plugging the injury. Secondary hemostasis refers to the deposition of insoluble fibrin, which is generated by the proteolytic coagulation cascade. This insoluble fibrin forms a mesh that is incorporated into and around the platelet plug. This mesh serves to strengthen and stabilize the blood clot. These two processes happen simultaneously and are mechanistically intertwined.
Platelets are small anuclear cell fragments that bud off from megakaryocytes, specialized large polyploid blood cells that originate in the bone marrow. Platelets are present at 150 to 400 million per milliliter of blood and circulate for about ten days. In a healthy blood vessel, and under normal blood flow, platelets do not adhere to surfaces or aggregate with each other. However, in the event of injury platelets are exposed to subendothelial matrix, and adhesion and activation of platelets begins. Multiple receptors on the surface of platelets are involved in these adhesive interactions, and these receptors are targeted by multiple adhesive proteins. The key for all of these receptors is that the adhesive interaction only takes place in the event of an injury to the blood vessel. This restriction is maintained in several different ways.
Secondary hemostasis consists of the cascade of coagulation serine proteases that culminates in cleavage of soluble fibrinogen by thrombin. Thrombin cleavage generates insoluble fibrin that forms a crosslinked fibrin mesh at the site of an injury. Fibrin generation occurs simultaneously to platelet aggregation. In intact and healthy blood vessels this cascade is not activated and several anticoagulant mechanisms prevent its activation. These include the presence of thrombomodulin and heparan sulfate proteoglycans on vascular endothelium. Thrombomodulin is a cofactor for thrombin that converts it from a procoagulant to an anticoagulant by stimulating activation of the anticoagulant serine protease protein C. Heparan sulfate proteoglycans stimulate the activation of the serine protease inhibitor (or serpin) antithrombin, which inactivates thrombin and factor Xa. Thrombin is the central serine protease in the coagulation cascade, and it executes several critical reactions. Thrombin critically cleaves fibrinogen to generate insoluble fibrin. Thrombin activates platelets via cleavage of PAR1 and PAR4. Thrombin is also responsible for positive feedback activation of coagulation that is critical for clot propagation.
Arterial thrombi are composed largely of aggregated platelets, whereas venous thrombi are composed more of fibrin with red blood cells enmeshed. The composition of these different thrombi is dictated by the different conditions in the arterial circulation and the venous circulation. One important aspect of this is blood flow, with higher flow rates and therefore higher sheer forces in the arterial circulation. Classically, arterial thrombosis and venous thrombosis are thought to have different risk factors. However, recent studies have suggested that some of the classic risk factors for arterial thromboses, such as obesity and high cholesterol, are also risk factors for venous thrombosis. The classic risk factors for venous thrombosis cause a hypercoagulable state and result in an increased tendency for activation of the coagulation cascade. These include acquired risk factors such as cancer, surgery, immobilization, fractures and pregnancy. Genetic risk factors include multiple variants in the coagulation cascade. The most common are factor V and prothrombin G20210A. Others are protein C or protein S heterozygosity and mutations in antithrombin.
The main bleeding disorders are genetically inherited. Hemophilia results from defects in secondary hemostasis. Hemophilia A is due to deficiency of factor VIII, and hemophilia B is due to deficiency of factor IX. Activated factor VIII and activated factor IX together form the intrinsic factor Xase complex on activated membrane surfaces that is critical in the positive feedback loop of blood coagulation. Therefore, deficiency in either of these proteins causes a very similar bleeding phenotype characterized by excess bruising, spontaneous bleeding into joints, muscles, internal organs and the brain. Factor VIII and factor IX are both X-linked genes. Thus hemophilia is primarily expressed in males, with hemophilia A present in about 1 in 5000 males and hemophilia B present in about 1 in 20,000 males. However, multiple mutations in either factor VIII or factor IX have been identified, and not all of them cause complete loss of protein or protein function. In fact, almost half of hemophilia A sufferers have de novo mutations that were not inherited from their parents. Depending on the mutation, hemophilia can be severe (<1% function), moderate (1–5%) or mild (5–20%).
Hemostasis has now been widely studied for more than a century. In that time we have generated a very detailed picture of the molecular and cellular events that play roles in normal and pathological hemostasis. But there are still many interesting questions that remain to be answered. This research has led to the development of numerous drugs that impact many different mechanisms of thrombosis or bleeding. Therefore, the prospect for continued improvements in patient health and quality of life is great.