clotting cascade

• The coagulation cascade is catalysed by protein enzymes known as coagulation ‘factors’. When activated, these factors trigger the conversion of other factors in the cascade resulting in secondary haemostasis.
• Secondary haemostasis refers to the stabilisation of activated platelets due to strands of fibrin -> generates a ‘stable mesh’ the main component in a thrombus (clot)
• The activation and adherence of platelets to the damaged endothelium (primary haemostasis), and vascular response to injury (e.g. vasoconstriction) are the other key components involved in thrombus formation and haemostasis.
• Fibrin is converted from its precursor fibrinogen by thrombin and is the end-product of the chain of reactions that make up the coagulation cascade.
• The coagulation cascade can be divided into the intrinsic, extrinsic and common pathways. Coagulation is more complex than the coagulation cascade, but is a useful starting point to understanding laboratory tests of coagulation.
• Coagulation factors are usually designated a number in Roman numerals with a letter ‘a’ after the numeral when in its active form. For example, activated factor X is referred to as factor Xa.

  Some factors are more commonly known by their name rather than their number. For example, factor II is usually referred to as thrombin.
• The extrinsic pathway:

  Initiation of the coagulation cascade is usually by tissue factor via the extrinsic pathway.

  Tissue factor is expressed on the surface of many cells found outside blood vessels but not on the surface of circulating blood cells or the endothelium. When the endothelium is damaged, tissue factor comes into contact with blood and combines with circulating factor VII to form a complex that leads to the activation of factor X, triggering the common pathway.
• The common pathway

  begins with activation of factor X (to factor Xa) -> via extrinsic or intrinsic pathway

  It is the final stage of the coagulation cascade and leads to the formation of thrombin and fibrin

  Factor Xa combines with factor V, platelet membrane phospholipids and Ca2+ ions to convert prothrombin into thrombin. Thrombin converts fibrinogen into fibrin strands -> form important structural component of a thrombus.

  thrombin also activates parts of the cascade by positive feedback through the intrinsic pathway -> large amounts of additional thrombin (a ‘thrombin burst’).
• • The intrinsic pathway 
  • activated by surface contact (when endothelial damage leads to exposure of clotting factors to negatively charged subendothelial surfaces) mediated by kallikrein
  • Within blood vessels, thrombin generated from previous activation of the extrinsic pathway also activates the intrinsic pathway
  • reactions in the intrinsic pathway -> initiation of the common pathway -> amplified thrombin production (positive feedback loop)
• what happens in the intrinsic pathway?

  Reactions in intrinsic pathway include:

  • Surface contact activates factor XII
  • Factor XIIa activates factor XI
  • Factor XIa activates factor IX
  • Factor IXa combines with factor VIIIa, platelet membrane phospholipid and Ca2+ ions to activate factor X
  • Factor Xa activates the common pathway generating more thrombin

  This continuous cycle means that vast amounts of thrombin can be generated from a single initial stimulus and a thrombus can be formed quickly after injury to limit blood loss.
• Actions of thrombin

  • Fibrin generation: thrombin converts fibrinogen -> fibrin
  • Intrinsic pathway activation: generates a positive feedback loop by intrinsic pathway
  • Factor XIII activation: thrombin converts factor XIII -> factor XIIIa (fibrin stabilising factor) which cross-links fibrin to generate a ‘fibrin mesh’ which encapsulates activated platelets creating a thrombus -> stopping bleeding
  • Platelet activation: thrombin receptors on platelets cause activation and aggregation -> enhance haemostatic effects of coagulation cascade
  • Regulation of clot formation
• An overview of the coagulation cascade

  here

  
• regulating the coagulation cascade:

  • protein C
  • antithrombin
  • heparin and fondaparinux
  • tissue factor pathway inhibitor
• Without regulatory mechanisms, the positive feedback loop triggered by thrombin in the intrinsic pathway would continue indefinitely and cause dangerous levels of clotting in the blood.
• Protein C is produced in response to thrombin binding to the receptor thrombomodulin on the vascular endothelium. This leads to protein S activation which then breaks down factors Va and VIIIa in a negative feedback loop.
• Antithrombin is a natural anticoagulant that is produced by the liver and destroys factors XIa, Xa and thrombin
• Heparin and fondaparinux enhance the natural effects of antithrombin in inhibiting thrombin and factor Xa.
• Tissue factor pathway inhibitor is a protein that binds to and inactivates factor VIIa and factor Xa.
• Vitamin K and Warfarin

  Warfarin is a vitamin K antagonist which prevents the synthesis of vitamin K-dependent clotting factors (II, VII, IX and X).

  Low molecular weight heparin (LMWH) is prescribed alongside warfarin when treatment is initiated. This is because vitamin K is also required for the synthesis of the natural anticoagulants protein C and protein S so Warfarin may inhibit their production initially. LMWH can be stopped once the INR is at the target range.
• The prothrombin time - time taken for fibrin to form via extrinsic pathway. Normal range is 9-12 seconds. Measures factor VII; factor X; factor V; prothrombin and fibrinogen.

  Causes of a prolonged PT include:

  • Disseminated intravascular coagulation (DIC)
  • Vitamin K deficiency (Factor VII is the vitamin K dependent clotting factor with the shortest half-life)
  • Chronic liver disease

  PT is the basis of INR (international normalised ratio - patient’s PT compared to a normal PT.

  INR monitors levels of anticoagulation in patients on Warfarin - adjust the dosage as necessary.
• Activated thromboplastin time (APTT)

  The time taken for fibrin to form via the intrinsic pathway.

  The normal range is 23-38 seconds. Factors measured include factor XII, factor XI, factor IX, factor VIII, factor X, factor V, prothrombin and fibrinogen.

  Causes of a prolonged APPT include:

  • Disseminated intravascular coagulation (DIC)
  • Clotting factor deficiencies (e.g. haemophilia A or B)
  • Lupus anticoagulant
  • Von Willebrand’s disease
• tests may not always accuratly represent the ability for blood to clot and therefore should be interpreted with caution, alongside the patient’s history and other investigations.

  For example, the PT assesses the extrinsic pathway after it has been initiated by tissue factor but does not account for the fact that in vivo (within circulating blood) large amounts of thrombin are produced via a positive feedback loop through the intrinsic pathway, with even small amounts of factor VII and this alone can bring about haemostasis.
• tests may not always accurately represent the ability for blood to clot and therefore should be interpreted with caution, alongside the patient’s history and other investigations.

  Another example is that the APTT can be interfered with by antibodies against phospholipids (Lupus anticoagulants) which prolong the APTT test but do not increase the tendency of a patient to bleed.
• • Haemophilia is a congenital deficiency in either factor VIII (haemophilia A), factor IX (haemophilia B) or factor XI (haemophilia C) leading to defective clotting and therefore a tendency to bleed.
• • Chronic liver disease can lead to defective coagulation as many coagulation factors are depleted as the liver’s synthetic function is impaired.
• • Factor V Leiden is a common genetic mutation to the gene coding for Factor V. It increases the resistance of factor V to activated protein C making it more difficult to break down and therefore leading to an increased tendency to clot.