Haemostasis

• Haemostasis is the arrest of bleeding from a broken blood vessel.
• Haemostasis is the first stage of acute wound healing and occurs in seconds to hours. It involves vasoconstriction, platelet aggregation and leucocyte migration
• The major steps in haemostasis are: vascular spasm, formation of platelet plug, conversion to a clot by reinforcement with fibrin, then tissue repair and gradual removal of clot
• Too little haemostasis leads to excessive bleeding and too much can lead to thrombus formation
• Vascular injury and spasm: damage to vessel wall exposes underlying collagen and chemical factors from damaged endothelial cells.
• Platelet activation: Platelet adhesion. Platelets adhere to the damaged collagen (binding sites include involvement of von Willebrand Factor). Platelets become activated.
• Platelet plug formation: Platelets become activated and release factors into surrounding area causing vasoconstriction and activating more platelets - positive feedback
• Coagulation cascade: Activated platelets stick to each other to form a platelet plug. Coagulation cascade is triggered.
• Recombinant F VII is used as a salvage treatment for intractable bleeding
• Coagulation is the formation of a blood clot, and is essential to haemostasis.

  The coagulation process is a cascade of events which lead to the formation of a blood clot. Proteins called clotting factors initiate reactions which activate more clotting factors.

  This process occurs via two pathways which unite downstream to form the common pathway. These are:

  • The extrinsic pathway: This is triggered by external trauma which causes blood to escape the circulation
  • The intrinsic pathway: This is triggered by internal damage to the vessel wall
• Extrinsic pathway:

  • Damage to the blood vessel means that factor VII exits the circulation into surrounding tissues
  • Tissue factor (factor III) is released by damaged cells outside the circulation
  • Factor VII and factor III form a complex, known as the TF-VIIa complex.
  • TF-VIIa then activates factor X into its active form, factor Xa
  • In conjunction with factor Va, this triggers the formation of thrombin.

  The extrinsic pathway is believed to be responsible for the initial generation of activated Factor X (Factor Xa), whereas the intrinsic pathway amplifies its production.
• Intrinsic pathway: longer and more intricate pathway:

  • Factor XII is activated once it comes into contact with negatively charged collagen on the damaged endothelium, triggering the cascade as detailed in Figure 1.
  • Along with clotting factors, platelets form a cellular ‘plug’ at the site of injury. These platelets also release mediators that facilitate further clotting, including Factor VIII.
  • Factor IX combines with Factor VIII to form an enzyme complex that activates factor X, which along with factor Va, stimulates the production of thrombin.
• Common Pathway of Coagulation

  The intrinsic and extrinsic pathways converge to give rise to the common pathway. The activated factor X causes a set of reactions resulting in the inactive enzyme prothrombin (also called factor II) being converted to its active form thrombin (factor IIa) by Prothrombinase.

  The thrombin then converts soluble fibrinogen (also referred to as factor I) into insoluble fibrin strands. The fibrin strands which comprise the clot are stabilised by factor XIII.
• To prevent excessive clotting and subsequent disease, mediators including Protein C and Protein S provide negative feedback on the clotting cascade.

  Protein C is activated following contact by thrombomodulin, which is itself activated by thrombin. Along with co-factors including protein S, activated protein C degrades factor Va and factor VIIIa, thus slowing the rate of clotting.
• Calcium ions play a role in regulation of clotting through their interaction with the activation of several clotting factors. Low levels of calcium are therefore inhibitory to the clotting cascade.
• Antithrombin is a protease inhibitor that degrades thrombin, factor IXa, factor Xa, factor XIa and factor XIIa. It is constantly active but can be activated further by a group of common anticoagulants known as heparins.
• Protein C deficiency can either be inherited through a range of mutations or can be acquired in disease states including sepsis and liver disease.

  Due to the reduction in protein C, the clotting cascade is under less inhibition. Consequently, patients are pre-disposed to abnormal and excessive clotting, leading to illnesses including deep vein thrombosis and stroke.
• In order for blood flow to be re-established as the blood vessel heals, the thrombus must eventually be degraded. During fibrinolysis, fibrin is dissolved leading to the consequent dissolution of the clot.

  The endothelial cells of the blood vessel wall secrete tissue plasminogen activators (tPAs) which convert the precursor plasminogen into plasmin. Plasmin cleaves the fibrin within the thrombus, leading to degradation.

  tPAs are released very slowly following trauma from the endothelial cells, and therefore there is a time delay until there is a sufficient concentration for fibrinolysis.
• Haemophilia is an inherited bleeding disorder caused by partial or total deficiency in specific clotting factors. There are 3 types:

  • Type A: deficiency in factor VIII
  • Type B: deficiency in factor IX
  • Type C: deficiency in factor XI

  Haemophilia A and B are both X-linked disorders and are therefore most prevalent in males. Since this condition impairs the body’s normal clotting response, people with haemophilia may bruise easily, bleed spontaneously or bleed for longer following surgery and injury.

  Therapy for these patients includes the replacement of the missing clotting factor.
• Abnormal or unwanted blood clots in the circulation can lead to infarction. In the brain this is known as an ischaemic stroke; in the heart, this is known as a myocardial infarction.

  In order to degrade the blood clot, one treatment option is thrombolysis. This involves using synthetic tPAs to artificially destroy the clot. One obvious, but common, side effect is unwanted bleeding at other sites.
• The platelet plug is reinforced by fibrin mesh
• Fibrinogen (soluble) forms fibrin (forms mesh) via thrombin
• The reinforced platelet plug with fibrin and red cells trapped within is called a clot
• Prostacyclin from intact endothelium inhibits platelet adhesion and aggregation
• Nitric oxide from intact endothelium inhibits platelet adhesion
• The liver synthesises bile salts, which increase absorption of vitamin K in the the GI tract, and therefore the concentration of Vit K in the blood. Through post-translational modifications and clotting factors also produced by the liver, end up with clotting factors in the blood.
• Warfarin is a vitamin K antagonist
• Removal of the clot: cell growth and cell division repairs the damaged vessel. Clot retracts and slowly dissolves. Enzyme plasmin trapped in the clot acts on fibrin to break it into soluble fragments. This takes a week.
• Abnormal platelet numbers, abnormal platelet functions and absence of any clotting factors can all cause a defect in blood clotting mechanism
• von Willebrand factor is crucial in binding platelets to damaged collagen at the site of vascular injury/
• Fibrin mesh is important in blood clotting mechanism to prevent erythrocytes from flowing out
• Plasmin is required for the removal of a clot
• Smooth intact endothelium keeps away the collagen, tissue factor and vWF
• Glycocalyx inhibits the adhesion of platelets
• Prostacyclin and nitric oxide inhibit platelet activation and aggregation
• CD39 or ecto-ADPases metabolises ADP
• Tissue plasminogen activator (tPA) dissolves fibrin
• Thrombomodulin binds thrombin - activates protein C (APC)
• Heparan sulfate - cofactor for antithrombin
• Antithrombin is an anticoagulant that inactivates thrombin and inactivates factor Xa and IXa