1. Hematology 425 Normal Coagulation & Hemostasis
Russ Morrison
December 4, 2006
11/27/2018

2. Coagulation & Hemostasis
Hemostasis is defined as the combination of cellular and biochemical events that function to keep blood liquid within the veins and arteries, prevent blood loss from injuries by the formation of thrombi and reestablish blood flow during the healing process
When this process is out of balance, thrombosis (clotting) or hemorrhage will threaten life
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3. Coagulation & Hemostasis
It is the role of physicians and the laboratory to investigate the major hemostasis systems including
Blood vessels
Blood cells
Plasma proteins
The role of the investigation is to prevent, predict, diagnose and manage hemostatic disease
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4. Overview of Hemostasis
For an overview of hemostasis, several factors must be discussed including
Vascular intima
Blood platelets
Erythrocytes
Neutrophils
Monocytes
Plasma coagulation system
Fibrinolysis
Each will be discussed as a separate component, but none act independently of the others or from the group
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5. Primary & Secondary Hemostasis
Desquamation and small injuries to blood vessels
Involves vascular intima and platelets
Rapid, short-lived response
Secondary
Large injuries to blood vessels and tissues
Involves platelets and the plasma coagulation system
Dealyed, long term response
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6. Primary & Secondary Hemostasis
Primary hemostasis mechanisms are triggered by small injuries to blood vessels (pinpricks) Or commonplace desquamation of dying or damaged endothelial cells
In primary hemostasis, the blood vessel contracts to seal the wound and platelets fill the open space to form a plug
While seeming to be trivial, defects in this process can cause fatal hemorrhagic disorders
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7. Primary & Secondary Hemostasis
Secondary hemostasis is triggered by primary hemostasis mechanisms and is necessary to control bleeding from large wounds incurred through trauma, surgery or dental procedures
The plasma coagulation system accomplishes secondary hemostasis by producing a fibrin thrombus
Vascular intima and platelets are usually associated with 1o hemostasis and coagulation and fibrinolysis with 2o hemostasis, BUT all systems work together during hemostatic events
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8. Vascular Intima - Structure
The innermost lining of blood vessels is a contiguous layer of endothelial cells
These endothelial cells for a smooth, unbroken surface that promotes fluid passage of blood and prevents turbulence that might activate platelets and plasma enzymes
A collagen-rich basement membrane and its surrounding layer of connective tissues support the endothelial cells
Fibroblasts occupy the connective tissue layer and produce collagen
Smooth muscle cells, intermixed with fibroblasts in arteries and arterioles (not veins, venules or capillaries), contract during primary hemostasis
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9. Vascular Intima - Procoagulant
Intimal cells and the environment they are in play a primary role in hemostasis
Any harmful local stimulus (mechanical or chemical) induces vasoconstriction in arteries and arterioles. The smooth muscle cells contract, the vascular lumen narrows (or closes) and blood flow to the injured site is minimized
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10. Vascular Intima - Procoagulant
Second, the basement membrane and subendothelial connective tissues of arteries and veins are rich in collagen, an elastic protein that binds and activates platelets.
Third, endothelial cells secrete vWF, a glycoprotein that is necessary for platelets to adhere to exposed subendothelial collagen in arterioles
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11. Vascular Intima - Procoagulant
Fourth, upon activation, endothelial cells secrete and coat themselves with P-selectin, an adhesion molecule that promotes platelet and leukocyte binding. Additional adhesion molecules (ICAMs and PECAMs) that promote leukocyte binding are also secreted by endothelial cells
Finally, subendothelial cells (smooth muscle cells and fibroblasts) support a constitutive surface protein called tissue factor
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12. Vascular Intima
Exposed tissue factor activates the plasma coagulation system through factor VII
Tissue factor also appears on the surface of endothelial cells and on blood-borne monocytes during inflammation
While damaged vessels have PRO-COAGULANT properties, intact vessels prevent intravascular thrombosis and have ANTI-COAGULANT properties that act by several mechanisms
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13. Vascular Intima - Anticoagulant
Endothelial cells are rhomboid and contiguous which provides a smooth inner surface that evens blood flow and prevents turbulance
Endothelial cells also synthesize prostacyclin, a platelet activation inhibitor derived from arachidonic acid-prostaglandin pathway
Prostacyclin prevents unnecessary or undesirable platelet activation in undamaged vessels
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14. Vascular Intima - Anticoagulant
Nitric oxide is synthesized in endothelial cells, vascular smooth muscle cells, neutrophils and macrophages
Nitric oxide regulates vasoconstriction and is essential in maintaining healthy arterioles
Heparan sulfate is a glycosaminoglycan that slows plasma coagulation by activating antithrombin, a coagulation regulatory protein
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15. Vascular Intima - Anticoagulant
Heparin is a pharmaceutical manufactured from pig gut tissues, that resembles heparan sulfate
Heparin is used as a therapeutic to prevent propogation of thrombi that cause coronary thrombosis, strokes, DVTs and pulmonary thrombotic emboli
Another important endothelial cell anticoagulant is tissue factor pathway inhibitor (TFPI)
TFPI inactivates coagulation factor VIIa in the presence of factor Xa and controls the tissue factor (extrinsic) coagulation pathway
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16. Vascular Intima – Anticoagulant
Finally, endothelial surface membranes contain thrombomodulin, a protein that activates the protein C pathway
The protein C pathway then regulates the plasma coagulation mechanism by digesting activated factors V and VIII
The coagulation pathways (intrinsic and extrinsic) will be discussed in detail later in this discussion
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17. Vascular Intima - Fibrinolytic
ECs support fibrinolysis with two secretions, TPA and PAI-1
Tissue plasminogen activator (TPA) binds to polymerized fibrin and triggers activation of bound plasminogen to plasmin which ultimately digests the thrombus and restores blood flow
Plasminogen activator inhibitor (PAI-1) inactivates TPA (and vice/versa)
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18. Vascular Intima
The significance of the vascular intima to coagulation and hemostasis is recognized, but difficult to assess or to measure functional integrity
Research is currently being directed toward evaluation of blood vessel disorders
Hopefully, in the near future, we will gain protocols to assess the integrity of ECs, smooth muscle cells, fibroblasts and their collagen matrix
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19. Procoagulant Intima Structure Procoagulant Property
Smooth muscle cells in arterioles and arteries
Induce vasoconstriction
Exposed basement membrane
Collagen binds to vWF and platelets
Damaged or activated endothelial cells
Secrete vWF
Exposed smooth muscle cells and fibroblasts
Secrete adhesion molecules: P-selectin, ICAMS, PECAMS
Endothelial cells in inflammation
Tissue factor is induced by inflammatory process
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20. Anticoagulant Intima
Anticoagulant properties of intact endothelial cells
Rhomboid, presenting a smooth, contiguous surface
Secrete platelet inhibitor, prostacyclin
Secrete vascular relaxing factor, nitric oxide
Secrete anticoagulant glycosaminoglycan, heparan sulfate
Secrete coagulation pathway regulator, TFPI
Maintain the protein C coagulation regulatory system activator, thrombomodulin on their surfaces
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21. Platelets
Platelets are complex, metabolically active cells that interact with their environment and initiate hemostasis
Platelets adhere, aggregate and secrete (table 42-3)
Adhesion is the property of binding to nonplatelet surfaces such as subendothelial collagen
Aggregation is the property of platelets binding to each other
Secretion of platelet granule contents occurs during both adhesion and aggregation
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22. Platelet Adhesion
Damaged or dead endothelial cells release their hold on the basement membrane leaving an area of turbulence that traps platelets
In arterioles (where blood flow is rapid), vWF bridges the physical distance between platelets and exposed subendothelial collagen creating bonds that seal the platelet to the vessel wall
In venules and veins, platelets directly bind the matrix proteins and vWF is unnecessary for platelet adhesion
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23. Platelet Adhesion Platelets only adhere for a few hours
During adhesion, pltlts may secrete growth factors that stimulate fibroblast mitosis and aid in repair of subendothelial tissue
Neighboring endothelial cells produce daughter cells that fill in the space occupied by the platelet, eventually forcing the platelet out
This released platelet lives and circulates to repair other sites
Adhesion of platelets is essential to life, individuals without good adhesion (vW disease) demonstrate mild to severe uncontrolled bleeding
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24. Platelet Aggregation
With moderate to severe injury, platelets bind to each other to form platelet plugs which block the injury site and prevent further blood loss (fig 42-4)
When pltlts aggregate, they expend their stored energy sources, lose membrane integrity and form an unstructured mass called a syncytium
Once aggregated, pltlts can not disaggregate
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25. Platelet Aggregation
In addition to plugging the site of injury, platelet aggregates shed microplatelet membrane particles rich in phospholipids and a variety of coagulation proteins
These particles provide a localized environment that supports plasma coagulation
Platelet aggregation is necessary to prevent blood loss from major wounds
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26. Platelet Aggregation
To aggregate, pltlts depend upon an ample supply of vWF and fibrinogen
Fibrinogen is an abundant coagulation protein that bridges natural platelet-to-platelet distances and encourages the formation of the platelet plug
Fibrinogen is also the main structural component of a fibrin clot
Fibrinogen deficiency (as vWF deficiency) is associated with hemorrhagic disease because both platelet aggregation and fibrin clot formation are deficient
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27. Platelet Secretion
When platelets aggregate, they secrete their granular contents
Granules contain
A number of clot-promoting molecules called procoagulants
Vasoconstrictors that cause blood vessels to contract
Agonists that recruit and activate neighboring platelets
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28. Platelet Secretion
Secretion occurs at the same time as adhesion and aggregation
The substances secreted by platelets are essential to the plasma coagulation process
In storage pool diseases, platelet granules contain diminished quantities of these substances
Patients with storage pool disease suffer from easy bruising and a tendency to bleed freely after trauma, during surgery and after dental procedures
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29. Platelet Secretion
There are relatively few tests available to measure platelet function
Count and platelet morphology provides minimal, quantitative information
A bleeding time screens platelet function as can the Dade-Behring PFA-100
Platelet aggregometry tests are semi-quantitative and technically demanding
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30. Hemostasis – Other Blood Cells
Erythrocytes, monocytes and lymphocytes also participate in hemostasis
RBCs add bulk and structural integrity to the fibrin clot
In inflammation, monocytes and lymphocytes provide surface-borne tissue factor that triggers coagulation
Leukocytes also have a series of membrane integrins and selectins that bind adhesion molecules and help stimulate the production of inflammatory materials that stimulate the wound healing process
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31. Plasma Coagulation
There are at least 16 glycoproteins that function together to form a fibrin clot
Most are trypsin-like enzymes called serine proteases
The plasma coagulation system is complex and translates a diminutive physical or chemical event into a profound life-saving event (clot formation)
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32. Plasma Coagulation
If even one of the plasma protein coagulants is missing in an individual, he/she is doomed to lifelong hemorrhage, chronic inflammation and transfusion dependence
The plasma coagulation system which will be discussed includes normal plasma coagulation, plasma coagulation control and fibrinolysis
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33. Plasma Coagulation - Nomenclature
Plasma contains a minimum of 16 procoagulants or clotting factors
Most are glycoproteins synthesized in the liver (a few are made by monocytes, ECs and megakaryocytes
Eight of the clotting factors are enzymes that circulate in an inactive form and are called zymogens
6 others are cofactors that bind and stabilize their respective enzymes
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34. Plasma Coagulation - Nomenclature
During the thrombosis process, the procoagulants become activated and produce a localized thrombus (clot)
At least 6 plasma glycoproteins serve as anticoagulants to keep the coagulation process regulated
In 1958, the International Committee for the Standardization of the Nomenclature of Blood Clotting Factors named the plasma procoagulants using roman numerals
The clotting factors were numbered in order of discovery
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35. Plasma Coagulation - Nomenclature
When a procoagulant becomes activated, a lower case “a” appears behind the roman numeral (activated factor VII becomes factor VIIa)
Both zymogens and cofactors become activated in the coagulation process
Table 42-5 lists the coagulation factors, their functions, molecular weight, half-life and normal plasma concentration
Knowledge of the coagulation factors (name and/or number) is essential clinical information that helps interpret lab tests and to design effective replacement therapies with deficiency-related hemorrhagic diseases
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36. Function of Plasma Procoagulants
Plasma procoagulants may be either serine proteases or cofactors
Serine proteases are proteolytic enzymes of the trypsin family and include the procoagulants thrombin (IIa), factors VIIa, IXa, Xa, Xia, and XIIa and prekallikrein
Each has a reactive seryl amino acid residue in its active site and acts upon its substrate by hydrolyzing peptide bonds and digesting the primary backbone to produce small polypeptide products
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37. Function of Plasma Procoagulants
These serine proteases are synthesized as inactive zymogens consisting of a single peptide chain
Activation occurs when the zymogen is cleaved at one or more specific sites by the action of another protease during the coagulation process
Activation is a localized process limited to the site of injury
Generalized plasma activation of zymogens is called DIC, a condition with high morbidity that is often fatal
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38. Plasma Procoagulant Serine Proteases
Inactive Zymogen
Active Protease
Cofactor
Substrate
Prothrombin (II)
VIIa
Fibrinogen
VII
Tissue factor
IX, X IX
IXa
VIIIa X Xa Va
Prothrombin XI
XIa
XII
XIIa
HMWK
Prekalikrein
Kalikrein
HMWK – High Molecular Weight Kininogen
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39. Function of Plasma Cofactors
Coagulation cofactors are tissue factor, factor V, factor VIII, and High Molecular Weight Kininogen (HMWK)
Each cofactor binds a particular serine protease
When the cofactor binds, the serine protease gains stability and increased reactivity
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40. Plasma Procoagulant Cofactors
Inactive Form
Active Form
Binds
Tissue Factor
Exposed tissue factor
VIIa V Va Xa
VIII
VIIIa
IXa
HMWK
Kinin
XIIa, prekallekrein
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41. Function of Add’l Components
Remaining components of the plasma coagulation pathway are factor XIII, fibrinogen, calcium, vWF and phospholipids
Vactor XIIIa is a transglutaminase that catalyzes the transfer of amino acids among the gamma chains of fibrin polymers
The reaction cross-links fibrin polymers to add physical strength to the fibrin clot
Factor XIII reacts with other plasma and cellular structural proteins and is essential for wound healing and tissue integrity
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42. Function of Add’l Components
Fibrinogen is the primary substrate of the coagulation pathway
When hydrolyzed by thrombin, fibrinogen polymerizes to form the primary structural protein of the fibrin clot
vWF is a large glycoprotein that participates in platelet adhesion and transports the procoagulant factor VIII
vWF is synthesized in megakaryocytes and endothelial cells
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43. Function of Add’l Components
Coagulation reactions occur on the surface of platelet or endothelial cell membrane phospholipids, not in the fluid phase
Serine proteases bind to negatively charged phospholipid surfaces through positively charged calcium ions
Therefore, calcium is involved in the majority of the coagulation pathway reactions
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44. Fibrinogen Structure & Fibrin Formation
Fibrinogen is the primary substrate of thrombin
It is a large glycoprotein synthesized in the liver
NL plasma concentration of fibrinogen is 200 to 400 mg/dL, the highest of all the plasma procoagulants
Platelet granules absorb, transport and release fibrinogen
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45. vWF/Factor VIII Complex
vWF is a multimeric glycoprotein composed of several subunits of 240,000 Da each
The subunits are produced by endothelial cells and megakaryocytes where they combine to form molecules of 800K to 20M Da
vWF molecules are stored in platelet alpha granules and in endothelial cells, where their storage sites are called Weibel-Palade bodies
vWF molecules are released from storage into the plasma, with NL concentration of 7-10 ug/ml
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46. vWF/Factor VIII Complex
vWF molecules carry receptor sites for platelets and collagen
The primary platelet receptor site binds a platelet surface integrin, glycoprotein 1b/IX and fills the space between the platelet and exposed subendothelial collagen during platelet adhesion
A secondary site binds another platelet integrin, glycoprotein IIb/IIIa
A third site binds plasma procoagulant cofactor, factor VIII
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47. vWF/Factor VIII Complex
Factor VIII is produced by the liver and from other tissues
Its source (as well as the source of factor IX) is the X chromosome
Factor VIII has a molecular mass of 260K Da and circulates bound to vWF
Free factor VIII is unstable and cannot be detected in plasma
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48. vWF/Factor VIII Complex
Factor VIII is a labile factor that deteriorates over a period of hours in stored blood even when still bound to vWF
Individuals with hemophilia A have diminished factor VIII activity, but normla vWF levels
Because factor VIII depends on vWF for stability, people with vWF disease have both diminished vWF and diminished factor VIII activity, but factor VII levels do not drop to clinically significant levels
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49. Vitamin K-Dependent Prothrombin Group
Prothrombin; factors VII, IX, and X and the regulatory proteins, protein C and protein S are all vitamin K dependent
This group is called the prothrombin group because of their structural resemblance to prothrombin
The net negative charge of these molecules enable them to bind ionic calcium
The bound calcium enables the vitamin K-dependent proteins to bind to phospholipids
Phospholipid binding is essential to coagulation reactions
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50. Vitamin K-Dependent Prothrombin Group
When vitamin K is deficient or a vitamin K antagonist is present, this group of plasma coagulation factors is released from the liver without the carboxylation that produces the net negative charge
Without the charge, they cannot bind to calcium and participate in the coatulation reaction
Vitamin K antagonism is the basis of oral anticoagulant therapy (warfarins/ coumadin)
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51. The Tissue Factor Pathway
Coagulation is triggered by the exposure of tissue factor to plasma proteins
Activated factor VII (VIIa) binds tissue factor in the presence of phospholipid and Ca and triggers the activation of the zymogens, beginning with factors IX and X
Activated factor IX (IXa) binds VIIIa on phospholipid surfaces and that complex activates factor X (fig 42-10)
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52. The Tissue Factor Pathway
Factor Xa binds vactor Va on phopholipid surfaces (fig and 42-12)
This Xa-Va complex activates prothrombin in a multistep hydrolytic process that releases a peptide fragment from prothrombin called prothrombin 1+2
Activated prothrombin is called thrombin
Thrombin cleaves fibrinopeptides A and B from plasma fibrinogen causing the formation of fibrin polymer
Fibrin polymer is stabilized by the cross-linking action of factor XIIIa (fig 42-7)
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53. The Tissue Factor Pathway
This tissue factor pathway is characterized by multimolecular complex formation
The first complex is composed of tissue factor, VIIa, phospholipid and Ca2+
The second is IXa, VIIIa, phospholipid and Ca2+ (also called tenase)
The third complex is Xa, Va, phospholipid and Ca2+ (also called prothrombinase)
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54. Coagulation Cascade Complexes
Components
Function
First complex
VIIa, tissue factor, phospho-lipid and Ca2+
Cleaves IX and X
Second complex
IXa, VIIIa, phospholipid,Ca2+
Cleaves X, called “tenase”
Third complex
Xa, Va, phospholipid and Ca2+
Cleaves pro-thrombin, called “prothrombinase”
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55. Factor XI Activation Pathway
Thrombin also activates factor XI
Activated XI (Xia) activates factor IX and the reaction proceeds as described in the prior slides
This alternative pathway is essential to normal coagulation as seen in case of moderate to severe hemorrhage in folks with factor XI deficiency (Rosenthal syndrome)
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56. Contact Factors
The “contact factor” complex composed of XIIa, HMWK and prekallikrein also activates factor XI
Contact factors do not have an in vivo procoagulant function, but respond to the presence of negatively charged particulate surfaces such as nonsiliconized glass or the aPTT test reagent
Foreign material, such as stents or valves may activate contact factors in vivo and cause thrombosis
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57. Extrinsic, Intrinsic and Common Pathways
Before 1992, coagulation experts called the activation of factor XII the primary step in coagulation
The reason was that factor XII could be found in blood and tissue factor could not be found in blood
The reaction steps that begin with factor XII and culminate in fibrin polymerization was called the intrinsic pathway
The coagulation factors of the intrinsic pathway, in order of reaction were XII, prekalikrein, HMWK, XI, IX, VIII, X, V, prothrombin and fibriogen
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58. Extrinsic, Intrinsic and Common Pathways
The tissue factor pathway was called the extrinsic pathway
The extrinsic pathway includes factors VII, X, V, prothrombin and fibrinogen
Neither factor VIII nor factor IX was included in the extrinsic pathway because their contribution was bypassed in the prothrombin time test (PT)
The PT was used to measure the integrity of the extrinsic pathway, while the aPTT was used to measure the intrinsic pathway
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59. Extrinsic, Intrinsic and Common Pathways
These two pathways had factors X, V, prothrombin, and fibrinogen in common
The steps involving these “common” factors was termed the common pathway
Terms intrinsic and extrinsic pathways no longer fit with current understanding of coagulation, but one must be aware of the terms as they are still used in clinical discussion of coagulation processes
See fig 42-10
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60. The coagulation cascade
The coagulation cascade. Legend: HMWK = High molecular weight kininogen, PK = Prekallikrein, TFPI = Tissue factor pathway inhibitor. Black arrow = conversion/activation of factor. Red arrows = action of inhibitors. Blue arrows = reactions catalysed by activated factor. Grey arrow = various functions of thrombin.
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61. Coagulation Regulatory Mechanisms
Fig illustrates the regulatory points of the coagulation mechamism
We will summarize the points and then discuss selected ones in more detail
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62. Coagulation Regulatory Mechanisms
Name
Function MM
(Da)
Half-life
(hours)
Mean plasma concen-tration
Tissue factor pathway in-hibitor
With Xa,binds TF-VIIa
33K
unknown
60-80 ng/mL
Thrombo-modulin
EC surface receptor for thrombin
450K
Does not circulate
None
Protein C
Serine Protease
62K
7-9
2-6 ug/mL
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63. Coagulation Regulatory Mechanisms
Name
Function MM
(Da)
Half-life
(hours)
Mean plasma concen-tration
Protein S
cofactor
75K
unknown
20-25 ug/mL
Antithrom-bin
SERPIN
58K 68
24-40 mg/dL
Heparin cofactor II
65k 60
30-70 ug/mL
α1-anti-trypsin
60K
250 mg/dL
α2-Macro-globulin
725K
,g/dL
MM = molecular mass, SERPIN = serine protease inhibitor
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64. Coagulation Regulatory Mechanisms
Tissue Factor Pathway Inhibitor
Fig & 14, TFPI inactivates factor VIIa
It is the action of TFPI that makes the tissue factor-factor VIIa reaction one with a short half-life
Coagulation pathway amplification would occur primarily through factor XI
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65. Coagulation Regulatory Mechanisms
Protein C Regulatory System, fig 42-15
During thrombosis, thrombin cleaves fibrinogen and activates factors V, VIII, XI and XIII
In intact normal vessels, where coagulation is unwanted, thrombin binds the EC membrane protein thrombomodulin and triggers the Protein C system, an anticoagulant regulatory system
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66. Coagulation Regulatory Mechanisms
The complex of thrombin-thrombomodulin activates the plasma zymogen, protein C
Activated protein C (APC) then binds free plasma protein S
The stabilized APC-protein S complex hydrolyzes and inactivates factors Va and VIIIa
Protein S is synthesized in the liver and circulates in the plasma in two forms
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67. Coagulation Regulatory Mechanisms
The two forms of circulating Protein S are:
Free
Covalently bound to the complement control protein, C4b-binding protein
Bound protein S cannot participate in the protein C anticoagulant pathway, only free plasma protein S
In inflammatory conditions the complexed Protein S increases which decreases free Protein S
Deficiencies of Proteins C & S may be associated with recurrent venous thromboembolic disease
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68. Coagulation Regulatory Mechanisms
Serine Protease Inhibitors (SERPINs), fig 42-16
Antithrombin is a SERPIN that binds and neutralizes thrombin and factors IXa, Xa, XIa, and XIIa
Anththrombin was the first of the plasma coagulation regulatory proteins to be identified and the first to be assayed routinely in the clinical laboratory
Other SERPINs include heparin cofactor II, α1-ahtitrypsin and α2-macroglobulin
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69. Coagulation Regulatory Mechanisms
Antithrombin and heparin cofactor II both require heparin for effective anticoagulant activity
Unfractionated therapeutic heparin is a heterogenous glycosaminoglycan
Therapeutic unfractionated heparin increases antithrombin’s ability to neutralize thrombin and other serine proteases by 1000 times and simultaneously binds thrombin and antithrombin
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70. Fibrinolysis The last stage of coagulation is fibrinolysis
Fibrinolysis begins a few hours after fibrin polymerization and cross-linking
The precise mechanism of fibrinolysis is the subject of current research, but in general, consists of systematic hydrolytic digestion of cross-linked fibrin olymers by bound plasmin (fig 42-17)
Bound plasminogen is activated by TPA several hours after thrombus formation and restores normal blood flow through repaired vessels
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71. Fibrinolysis - Plasminogen
Plasminogen is a 90K Da zymogen produced in the liver
Plasminogen becomes bound to fibrin at the time of fibrin polymerization
Bound plasminogen becomes converted to an active plasmin molecule through proteolysis
The active form, plasmin, is a serine protease that systematically digests the fibrin polymer
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72. Fibrinolysis - Plasminogen
Plasmin is also capable of digesting fibrinogen and factors V and VIII, but is localized to and bound to fibrin which prevents that activity
ECs secrete tissue plasminogen activator (TPA), which hycrolizes bound plasminogen and initiates fibrinolysis
TPA, forms covalent bonds with fibrin during polymerization and segregates at the surface of the thrombus with plasminogen where it begins the digestion process over time
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73. Fibrinolysis - Plasminogen
Free TPA circulates bound to nhibitors such as PAI-1 and is cleared from plasma
Synthetic recombinant TPA is a therapeutic drug widely used to dissolve pathologic clots that form in venous and arterial thrombotic disease
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74. Fibrinolysis - Urokinase
Urinary tract epithelial cells secrete an “intrinsic” plasminogen activator called urokinase
Small amounts of urokinase circulate in the plasma and become incorporated into the mix of fibrin-bound plasminogen and TPA at the time of thrombus formation
Urokinase does not bind firmly to fibrin and has a minor physiologic effect
Like TPA, urokinase is widely used to dissolve clots in heart attacks, strokes and DVTs
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75. Fibrinolysis – Plasminogen Activator Inhibitor-1
TPA and urokinase are prevented from activating free, fluid-phase plasminogen by an inhibitor, PAI-1
PAI-1 is secreted from endothelium with TPA
Only at times of endothelial cell activation, such as following trauma, does the level of TPA secretion exceed that of PAI-1 to initiate fibrinolysis
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76. Fibrinolysis – α2-antiplasmin
Bound plasmin digests clots and restores blood vessel patency
Free plasmin digests plasma fibrinogen, factor V, factor VIII, and fibronectin, causing potentially fatal primary fibrinolysis
α2-antiplasmin is a free plasma protein that rapidly and irreversibly binds free plasmin
α2-antiplasmin also becomes cross-linked to fibrin during polymerization rendering fibrin resistant to digestion by plasmin
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77. Fibrin Degradation Products and D-dimer
Fibrinolysis proceeds in a methodical fashion, producing a series of identifiable fibrin fragments (X,Y,D,E, D-D)
Several of these fragments inhibit hemostasis by preventing platelet activation and by hindering fibrin polymerization
These fragments can be detected by laboratory assay
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78. Fibrin Degradation Products and D-dimer
The D-D fragment is called D-dimer and is separately detectable by immunoassay to provide evidence of cross-linking
Pathologic degradation of fibrinogen, as occurs when free plasmin is present, yields D and E fragments but no detectable D-dimer
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