Hemostasis - Advanced

Hemostasis is the process by which platelets, clotting factors, and endothelial cells produce clots to stop bleeding. The body is constantly working to maintain a balance between coagulation, which is necessary to prevent vascular leakage, and excessive clotting, which presents hazards to blood flow and tissue perfusion. 1. Vascular spasm occurs immediately after vessel damage and serves to slow blood flow and bleeding. It is not usually sufficient to stop vessel leakage, but it allows time for more extensive interventions. 2. Platelet plugs form as the result of platelet activation and aggregation; this happens quickly. Thousands of time each day, small tears form in the vasculature and are effectively closed by platelet plugs. 3. Coagulation reinforces the plug by forming a clot comprised of platelets and fibrin mesh with trapped blood cells. Adhesion, Activation, and Aggregation Disc-shaped inactive platelets with granules are pulled to the edges of the vessel. Adhesion occurs mainly via bridging by von Willebrand Factor. Platelet receptor glycoprotein Ib binds to von Willebrand Factor, which binds to exposed collagen fibers. Platelets can also adhere to collagen directly via alpha-2-beta-1 integrins; other platelet integrins bind to subendothelial fibronectin and laminin (we've omitted this for simplicity). Adhesion triggers platelet activation. Platelets transform form disc-shaped structures to "spiky" structures with filopodia; this morphological change increases platelet surface area. Upon activation, platelets release hundreds of protein types including Thrombin, ADP, and thromboxane A2 to promote additional platelet recruitment and activation; as we'll see, thrombin also plays a key role in coagulation. Other important molecules released include co-factor V, which we'll see again in the coagulation cascade, and, Von Willebrand Factor, which, as we've seen, is vital for platelet adhesion and subsequent activation. ADP upregulates expression of the alpha-IIb beta-3 integrin on activated platelets; this receptor binds with fibrinogen, which acts as a bridge between adjacent platelets. Pharmacological correlation: Aspirin limits platelet production of thromboxane A2, thus limiting platelet recruitment and formation of platelet plugs. After the plug is formed, and if vascular injury warrants clot formation, the coagulation cascade begins within seconds to minutes. The coagulation cascade comprises a series of amplifying enzymatic reactions that result in the deposition of an insoluble fibrin clot; the production of thrombin, which converts fibrinogen to fibrin, is integral to this process. Calcium ions, which are required for several key reaction steps. Coagulation factors, which are the driving forces of the cascade; they comprise proenzyme factors are transformed to active enzymes, and are signified by the lower case "a." Many coagulation factors are synthesized by the liver; thus, coagulation disorders are associated with liver disease. Co-factors are reaction accelerators; they form complexes with activated coagulation factors and calcium ions. Traditionally, these pathways have been categorized as "intrinsic" and "extrinsic," but modern models emphasize their cooperation in thrombin formation. 1. Endothelial rupture exposes tissue factor to the blood; because tissue factor is exogenous to the blood, it is the start of the so-called "extrinsic" pathway (tissue factor is also known as thromboplastin or CD142). 2. Tissue factor activates coagulation factor VII, and, with calcium, forms a complex that activates factor IX (this complex is sometimes called extrinsic tenase). 3. Activated factor IX, with calcium and co-factor VIIIa, activates factor X. 4. Activated factor X combines with calcium and co-factor Va to form prothrombinase, which is the complex that transforms prothrombin (aka, factor II) to thrombin (factor IIa). 5. Thrombin converts fibrinogen to fibrin, which forms a meshwork that surrounds the aggregated platelets and traps other materials, including red and white blood cells and serum. Aggregated platelets influence thrombin production; because all the elements in this pathway are inherent to the blood, this is traditionally called the "intrinsic" pathway 1. The negative surface of activated platelets activates Factor XII (aka, Hageman factor). 2. Factor XIla activates Factor XI. 3. Factor XIa activates Factor IX, which, as we've seen, forms a complex that ultimately leads to the production of thrombin and conversion of fibrinogen to fibrin. Thrombin has other, indirect ways of promoting clotting: It activates co-factors VIII and V, and factor XI; thus, it promotes its own production. Thrombin also ensures the stability of the clot: show that it activates factor XIII, aka, fibrin stabilizing factor, which, as its name implies, strengthens the bonds of the fibrin mesh of the clot. Thrombin also acts outside of the coagulation cascade via protease activation receptors (PARs): Promotes platelet activation and release of thromboxane A2 for platelet plug formation. Up-regulates pro-inflammatory mediators, such as cytokines and chemokines, implicating it in inflammatory conditions. Soon after the clot is formed, retraction occurs: the platelets contract and expel serum. Serum lacks fibrinogen and clotting factors, so it cannot coagulate. Fibrin fibers absorb thrombin, which prevents excessive clotting. Unabsorbed thrombin combines with anti-thrombin III, which neutralizes it. Heparin, which is normally circulating in small concentrations but can be administered in higher doses, increases the effectiveness of anti-thrombin III and removes factors IX, X, XI, XII. Thus, it is prescribed to prevent thrombosis. Healthy, intact endothelial cells produce nitric oxide, prostacyclin, and tissue plasminogen activator (t-PA) to prevent platelet adhesion and clot formation. Once the vessel wall heals, the clot must be removed via fibrinolysis, the breakdown of the fibrin mesh. Plasminogen binds to the fibrin; plasminogen is a proenzyme produced by the liver. Endothelial cells release tissue plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA), which bind to fibrin and cleave plasminogen to form active plasmin. Recall that thrombin itself is a stimulus for t-PA release. Plasmin breaks down fibrin, fibrinogen, and some coagulation factors to dissolve the clot. In a positive feedback cycle, plasmin also converts t-PA and u-PA into their more active forms, which promotes further fibrinolysis. Fibrinolysis produces fibrin degradation products (FDPs), most notably D-dimer, which is measured to assess thrombotic states (an elevated result may indicate excessive thrombotic states). When not bound to fibrin, plasminogen and plasmin activity is inhibited: Unbound t-PA and u-PA are inhibited by plasminogen activator inhibitor 1 (PAI-1). Alpha-2 antiplasmin inhibits unbound plasmin. Vitamin K is critical to hepatic synthesis of prothrombin and factors VII, IX, and X. Thus, vitamin K antagonists, such as warfarin, are prescribed to inhibit clot formation in patients at risk of thrombosis.