Coagulation

The coagulation of blood is a complex process during which blood forms solid clots. It is an important part of haemostasis (the cessation of blood loss from a damaged vessel) whereby a damaged blood vessel wall is covered by a fibrin clot to stop hemorrhage and aid repair of the damaged vessel. Disorders in coagulation can lead to increased hemorrhage and/or thrombosis and embolism.

Coagulation is extremely similar in all mammals, with all mammals using a combined cellular and serine protease mechanism. The system in humans is the most extensively researched and therefore the best known. This article focuses on human blood coagulation.

In brief

In a normal individual, coagulation is initiated within 20 seconds after an injury occurs to the blood vessel damaging the endothelial cells. Platelets immediately form a haemostatic plug at the site of injury. This is called primary haemostasis. Secondary haemostasis then follows—plasma components called coagulation factors respond (in a complex cascade) to form fibrin strands which strengthen the platelet plug. Contrary to popular belief, coagulation from a cut on the skin is not initiated by air or drying out, but by platelets adhering to and activated by collagen in the blood vessel endothelium. The activated platelets then release the contents of their granules, these contain a variety of substances that stimulate further platelet activation and enhance the haemostatic process.

The use of adsorbent chemicals, such as zeolite, and other haemostatic agents is also being explored for use in sealing severe injuries quickly.

Primary haemostasis

Primary haemostasis is initiated when platelets adhere, using a specific platelet collagen receptor glycoprotein Ia/IIa, to collagen fibers in the vascular endothelium. This adhesion is mediated by von Willebrand factor (vWF), which forms links between the platelet glycoprotein Ib/IX/V and collagen fibrils.

The platelets are then activated and release the contents of their granules into the plasma, in turn activating other platelets and white blood cells. The platelets undergo a change in their shape which exposes a phospholipid surface for those coagulation factors that require it. Fibrinogen links adjacent platelets by forming links via the glycoprotein IIb/IIIa. In addition, thrombin activates platelets.

Secondary haemostasis

The coagulation cascade

The coagulation cascade of secondary hemostasis has two pathways, the Contact Activation pathway (formerly known as the Intrinsic Pathway) and the Tissue Factor pathway (formerly known as the Extrinsic pathway) that lead to fibrin formation. It was previously thought that the coagulation cascade consisted of two pathways of equal importance joined to a common pathway. It is now known that the primary pathway for the initiation of blood coagulation is the Tissue Factor pathway. The pathways are a series of reactions, in which a zymogen of a serine protease and its glycoprotein co-factor are activated to become active components that then catalyze the next reaction in the cascade. Coagulation factors are generally indicated by Roman numerals, with a lowercase a appended to indicate an active form, ultimately resulting in cross-linked fibrin.

The coagulation factors are serine proteases (enzymes) except FVIII and FV which are glycoproteins. The serine proteases act by cleaving other proteins at specific sites. Factor XIII is a transglutaminase. Protein C is also a serine protease. The coagulation factors circulate as inactive zymogens.

The coagulation cascade can be summarised as follows: -

Tissue Factor pathway: the main role of the tissue factor pathway is to generate a "thrombin burst," thrombin being the single most important constituent of the coagulation cascade in terms of its feedback activation roles. FVIIa circulates in a higher amount than any other activated coagulation factor and following damage to the blood vessel endothelium Tissue Factor (TF) is released, this then forms a complex with FVIIa (TF-FVIIa) this activates FIX and FX. FVII itself is activated by thrombin, FXIa, plasmin, FXII and FXa. The activation of FXa by TF-FVIIa is almost immediately inhibited by tissue factor pathway inhibitor (TFPI). FXa and its co-factor FVa form the prothombinase complex which activates prothrombin to thrombin. Thrombin then activates other components of the coagulation cascade, including FV and FVII (which activates FXI which in turn activates FIX), and activates and releases FVIII from being bound to vWF. FVIIIa is the co-factor of FIXa and together they form the "tenase" complex which activates FX and so the cycle continues.
Contact Activation pathway: There is formation of the primary complex on collagen by high molecular weight kininogen (HMWK), prekallikrein and FXII (Hageman factor). Prekallikrein is converted to kallikrein and FXII becomes FXIIa. FXIIa converts FXI into FXIa. Factor XIa activates FIX, which with its co-factor FVIIIa form the tenase complex which activates FX to FXa. The minor role that the contact activation pathway has in initiating clot formation can be illustrated by the fact that patients with severe deficiencies of FXII, HMWK and prekallikrein do not have a bleeding disorder.
Thrombin Thrombin has a large array of functions. Its primary role is the conversion of fibrinogen to fibrin, the building block of a haemostatic plug. In addition, it activates Factors VIII and V and their inhibitor protein C (in the presence of thrombomodulin), and it activates Factor XIII, which forms covalent bonds that crosslink the fibrin polymers that form from activated monomers.

Following activation by the contact factor or tissue factor pathways the coagulation cascade is maintained in a prothrombotic state by the continued activation of FVIII and FIX to form the tenase complex, until it is down regulated by the anticoagulant pathways.

Cofactors and inhibitors

Various substances are required for the proper functioning of the coagulation cascade:

Calcium and phospholipid (a platelet membrane constituent) are required for the tenase and prothrombinase complexes to function. Calcium mediates the binding of the complexes via the terminal gamma-carboxy residues on FXa and FIXa to the phospholipid surfaces expressed by platelets. Calcium is also required at other points in the coagulation cascade.
Vitamin K is an essential factor to a hepatic gamma-glutamyl carboxylase that adds a carboxyl group to glutamic acid residues on factors II, VII, IX and X, as well as Protein S, Protein C and Protein Z. Deficiency of vitamin K (e.g. in malabsorption), use of inhibiting anticoagulants (warfarin, acenocoumarol and phenprocoumon) or disease (cirrhosis, hepatocellular carcinoma) impairs the function of the enzyme and leads to the formation of PIVKA's (proteins formed in vitamin K absence) this causes partial or non gamma carboxylation and affects the coagulation factors ability to bind to expressed phospholipid.

Three mechanisms keep the coagulation cascade in check. Abnormalities can lead to an increased tendency toward thrombosis:

Protein C is an important co-factor inhibitor, which degrades the co-factors FVa and FVIIIa. It is activated by thrombin with thrombomodulin and requires its co-enzyme Protein S to function. Quantitative or qualitative deficiency of either may lead to thrombophilia (a tendency to develop thrombosis). Impaired action of Protein C (activated Protein C resistance), for example by having the "Leiden" variant of Factor V or high levels of FVIII also may lead to a thrombotic tendency.
Antithrombin is a serine protease inhibitor (serpin) that degrades the serine proteases; thrombin and FXa, as well as FXIIa, and FXIa. It is constantly active, but its adhesion to these factors is increased by the presence of heparan sulfate (a glycosaminoglycan) or the administration of heparins (different heparinoids increase affinity to F Xa, F IIa, or both). Quantitative or qualitative deficiency of antithrombin (inborn or acquired, e.g. in proteinuria) leads to thrombophilia.
Tissue factor pathway inhibitor (TFPI) inhibits F VIIa-related activation of F IX and F X after its original initiation.

Testing of coagulation

Numerous tests are used to assess the function of the coagulation system:

Common: aPTT, INR (PT), TCT, bleeding time, D-dimer
Other: mixing test (whether an abnormality corrects if the patient's plasma is mixed with normal plasma), antiphosholipid antibodies, coagulation factor assays, genetic tests (eg. factor V Leiden, prothrombin mutation G20210A), dilute Russell viper venom test (dRVVT), platelet function tests, thromboelastography (TEG) or thromboelastometry (ROTEG).

The contact factor pathway is initiated by activation of the "contact factors" of plasma, and can be measured by the activated partial thromboplastin time (aPTT) test.

The Tissue factor pathway is initiated by release of "tissue factor" (a specific cellular lipoprotein), and can be measured by the prothrombin time (PT) test. This is reported as an INR value when used for the dosing of oral anticoagulants such as warfarin.

The quantatative and qualitative screening of fibrinogen is measured by the thrombin time (TCT). Measurement of the exact amount of fibrinogen present in the blood is generally done using the Clauss method for fibrinogen testing. Many analysers are capable of measuring a "derived fibrinogen" level from the graph of the Prothrombin time clot.

If a coagulation factor is part of the contact or tissue factor pathway, a deficiency of that factor will affect only one of the tests: thus hemophilia A, a deficiency of factor VIII, which is part of the contact factor pathway, results in an abnormally prolonged aPTT test but a normal PT test. The exceptions are prothrombin, fibrinogen and some variants of FX which can only be detected by either aPTT or PT.

Deficiencies of fibrinogen (quantitative or qualitative) will affect all screening tests.

Disorders of hemostasis

Disorders of the platelet and vessel wall
- Immune thrombocytopenic purpura (ITP)
- Thrombotic thrombocytopenic purpura (TTP)
- Hemolytic-uremic syndrome (HUS)
- Glanzmann's thrombasthenia
- Bernard-Soulier syndrome (abnormal glycoprotein Ib-IX-V complex)
- Storage pool disorders
- Paroxysmal nocturnal hemoglobinuria
- Gray platelet syndrome: deficient alpha granules.
- Delta storage pool deficiency: deficient dense granules.
Disorders of coagulation and thrombosis
- Disseminated intravascular coagulation
- Factor deficiencies
  - Hemophilia A (Factor VIII deficiency)
  - Hemophilia B (Factor IX deficiency, "Christmas disease")
  - Hemophilia C (Factor XI deficiency, mild bleeding tendency)
  - Von Willebrand disease (the most common bleeding disorder)
- Factor inhibitors
Disorders predisposing to thrombosis (see: hypercoagulability)
- Heparin-induced thrombocytopenia and thrombosis ("white clot syndrome")
- Antiphospholipid syndrome
  - Lupus anticoagulant
  - Anticardiolipin antibody
- Factor V Leiden
- Prothrombin mutation
- Protein C deficiency
- Protein S deficiency
- Antithrombin deficiency
- Abnormally raised levels of Factor VIII and Factor XI

Coagulation factors

Coagulation factors and related substances
Number and/or name	Function
I (fibrinogen)	Forms clot (fibrin)
II (prothrombin)	Its active form (IIa) activates I, V, VII, XIII, protein C, platelets
Tissue factor	Co-factor of VIIa (formerly known as factor III)
Calcium	Required for coagulation factors to bind to phospholipid (formerly known as factor IV)
V (proaccelerin, labile factor)	Co-factor of X with which it forms the prothrombinase complex
VI	Unassigned – old name of Factor Va
VII (stable factor)	Activates IX, X
VIII (antihemophilic factor)	Co-factor of IX with which it forms the tenase complex
IX (Christmas factor)	Activates X: forms tenase complex with factor VIII
X (Stuart-Prower factor)	Activates II: froms prothrombinase complex with factor V
XI (plasma thromboplastin antecedent)	Activates XII, IX and prekallikrein
XII (Hageman factor)	Activates prekallikrein and fibrinolysis
XIII (fibrin-stabilizing factor)	Crosslinks fibrin
von Willebrand factor	Binds to VIII, mediates platelet adhesion
prekallikrein	Activates XII and prekallikrein; cleaves HMWK
high molecular weight kininogen (HMWK)	Supports reciprocal activation of XII, XI, and prekallikrein
fibronectin	Mediates cell adhesion
antithrombin III	Inhibits IIa, Xa, and other proteases;
heparin cofactor II	Inhibits IIa, cofactor for heparin and dermatan sulfate ("minor antithrombin")
protein C	Inactivates Va and VIIIa
protein S	Cofactor for activated protein C (APC, inactive when bound to C4b-binding protein)
protein Z	Mediates thrombin adhesion to phospholipids and stimulates degradation of factor X by ZPI
Protein Z-related protease inhibitor (ZPI)	Degrades factors X (in presence of protein Z) and XI (independently)
plasminogen	Converts to plasmin, lyses fibrin and other proteins
alpha 2-antiplasmin	Inhibits plasmin
tissue plasminogen activator (tPA)	Activates plasminogen
urokinase	Activates plasminogen
plasminogen activator inhibitor-1 (PAI1)	Inactivates tPA & urokinase (endothelial PAI)
plasminogen activator inhibitor-2 (PAI2)	Inactivates tPA & urokinase (placental PAI)
cancer procoagulant	Pathological factor X activator linked to thrombosis in cancer

History

Theories on the coagulation of blood have existed since antiquity. Physiologist Johannes Müller (1801-1858) described fibrin, the substance of a thrombus. Its soluble precursor, fibrinogen, was thus named by Rudolf Virchow (1821-1902), and isolated chemically by Prosper Sylvain Denis (1799-1863). Alexander Schmidt suggested that the conversion from fibrinogen to fibrin was the result of an enzymatic process, and labeled the hypothetical enzyme "thrombin" and its precursor "prothrombin"Shapiro SS. Treating thrombosis in the 21st century. N Engl J Med 2003;349:1762-4. PMID 14585945..

Platelets were identified in 1865, and their function was elucidated by Giulio Bizzozero in 1882Brewer DB. Max Schultze (1865), G. Bizzozero (1882) and the discovery of the platelet. Br J Haematol 2006;133:251-8. DOI 10.1111/j.1365-2141.2006.06036.x..

The theory that thrombin was generated by the presence of tissue factor and calcium was consolidated by Paul Morawitz in 1905. At this stage, it was known that thrombokinase/thromboplastin (factor III) was released by damaged tissues, reacting with prothrombin (II), which, together with calcium (IV), formed thrombin, which converted fibrinogen into fibrin (I)Giangrande PL. Six characters in search of an author: the history of the nomenclature of coagulation factors. Br J Haematol 2003;121:703-12. PMID 12780784..

The remainder of the biochemical factors in the process of coagulation were largely discovered in the 20th century.

A first clue as to the actual complexity of the system of coagulation was the discovery of proaccelerin (initially and later called Factor V) by Paul Owren (1905-1990) in 1947. He also postulated that its function was the generation of accelerin (Factor VI), which later turned out to be the activated form of V (or Va); hence, VI is not now in active use.

Factor VII (also known as serum prothrombin conversion accelerator or proconvertin, precipitated by barium sulfate) was discovered in a young female patient in 1949 and 1951 by different groups. Factor VIII turned out to be deficient in the clinically recognised but etiologically elusive hemophilia A; it was identified in the 1950s and is alternatively called antihemophilic globulin due to its capability to correct hemophilia A.

Factor IX was discovered in 1952 in a young patient with hemophilia B named Stephen Christmas (1947-1993). His deficiency was described by Dr. Rosemary Biggs and Professor R.G. MacFarlane in Oxford, UK. The factor is hence called Christmas Factor or Christmas Eve Factor. Christmas lived in Canada, and campaigned for blood transfusion safety until succumbing to transfusion-related AIDS at age 46. An alternative name for the factor is plasma thromboplastin component, given by an independent group in California.

Hageman factor, now known as factor XII, was identified in 1955 in an asymptomatic patient with a prolonged bleeding time named of John Hageman. Factor X, or Stuart-Prower factor, followed, in 1956. This protein was identified in a Ms. Audrey Prower of London, who had a lifelong bleeding tendency. In 1957, an American group identified the same factor in a Mr. Rufus Stuart. Factors XI and XIII were identified in 1953 and 1961, respectively.

The usage of Roman numerals rather than eponyms or systematic names was agreed upon during annual conferences (starting in 1955) of hemostasis experts. This committee evolved into the present-day International Committee on Thrombosis and Hemostasis (ICTH). Assignment of numerals ceased in 1963 after the naming of Factor XIII. The names Fletcher Factor and Fitzgerald Factor were given to further coagulation-related proteins, namely prekallikrein and high molecular weight kininogen respectively.

Factors III and VI are unassigned, as thromboplastin was never identified, and actually turned out to consist of ten further factors, and accelerin was found to be activated Factor V.

All mammals have an extremely closely related blood coagulation process, using a combined cellular and serine protease process. In fact, it is possible for any mammalian coagulation factor to "cleave" its equivalent target in any other mammal. The only nonmammalian animal that uses serine proteases for blood coagulation is the Horseshoe Crab.

References

Hematology | Coagulation system

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