Overview of coagulation Flashcards
1
Q
Secondary vs primary hemostasis
A
- Secondary: fibrin plug formation via coagulation cascade (joints and muscles)
- Primary: platelet plug formation via adhesion, activation, and aggregation (mucosa, ecchymosis, petechiae)
2
Q
Defects in secondary hemostasis
A
- Factor defeciency (ex: factor VIII deficiency is hemophilia A- X linked)
- Increased fibrinolysis
- Decreased inhibition of coagulation
- Vit K deficiency (affects II, VII, IX, X; factor V will be normal)
- Liver disease (decreases all factor except factor VIII- made in endothelium)
- Use lab tests: PT, aPTT, thrombin time, fibrinogen level, 50/50, factor assays
3
Q
Defects in primary hemostasis
A
- Thrombocytopenia (decreased production, ineffective production, increased destruction)
- The cause of thrombocytopenia parallels the cause of anemia (i.e. if the anemia is due to hemolysis, then thrombocytopenia is also due to increased destruction)
- Splenic sequestration (normally 30% of all platelets are in the spleen)
- Abnormally high amounts of platelets in spleen leads to splenomegaly
- Platelet function defects (i.e. missing or mutated receptors)
- vWD (vonWillibrand’s disease) leads to primary hemostatic defects
- Dilution of platelets (many transfusions)
- Vascular defects
- Lab tests: platelet count, PFA100 (or bleeding time), vWF:Ag
4
Q
Thrombin time
A
- Test to only look at conversion of fibrinogen to fibrin by thrombin
- Only tells you if there is low fibrinogen, abnormal fibrinogen, or an inhibitor of thrombin (heparin, Ig, paraproteins, FDPs, ect)
- Doesn’t distinguish btwn these results, so must do fibrinogen level assay to see if there is a quantitative defect (hypofibrinogenemia)
5
Q
Prothrombin time (PT)
A
- PT test only looks at the extrinsic + common coagulation pathways
- This test is insensitive to thrombin inhibitors like heparin, since you overwhelm the system w/ tissue factor (TF) to induce coagulation
- Tests for the availability/function of factors VII, V, X, and II
- Also will be prolonged w/ hypo/dysfibrinogenemia (qualitative defects in fibrinogen)
- If PT test and aPTT are both prolonged, then may have defect in common pathway
- If PT test is prolonged but aPTT is normal then think factor VII problem (deficiency or Ab)
- Do a 50/50 mix to identify if its deficiency or inhibitor (Ab)
6
Q
Activated partial thromboplastin time (aPTT)
A
- Looks only at intrinsic + common coagulation pathways
- Is sensitive to heparin since there is no overwhelming stimulus, like in PT test
- The aPTT can be prolonged due to XII/HMWK/PK (prekallikrein) deficiencies, but these will not cause bleeding
- If PT and aPTT are both prolonged, think common pathway problem
- If PT is normal but aPTT is prolonged, think factor XI, IX, or VIII problem
- Deficiency vs inhibitor can be distinguished by doing 50/50
- In vWD there will be prolonged aPTT b/c lack of vWF leads to clearance of VIII (VIII requires vWF for stability)
7
Q
Platelet plug formation 1
A
- Starts w/ adhesion: exposure of sub endothelial collagen and vWF release due to damaged vessel, vWF binds to the exposed collagen
- vWF binds to GPIb/IX receptor on platelet
- Collagen binds to the GPIa/IIa and GPVI receptors on the platelet
- These binding processes are energy independent
- Activation occurs upon collagen, vWF, or thrombin binding to platelet
- Thrombin binds to PARs (protease activated receptors) on platelet membrane and contributes to hemostasis
8
Q
Platelet plug formation 2
A
- Activated platelets use energy dependent processes that result in: actin-induced shape change, rearrangement of cell membrane phospholipids (phosphatidyl serine to outer membrane), granule release, and adjustment of membrane receptors
- Platelets express receptor IIb/IIIa, which binds vWF (in the arteries) or fibrinogen (in the veins) and causes aggregation
- Platelets also express P-selectin to promote WBC migration to site of injury (binds to WBC PSGL-1), and facilitate the fusion of microvesicle w/ platelet membrane (lets platelets present TF on their membrane)
- Platelets release granules: dense (ADP, serotonin, Ca), alpha (fibrinogen, vWF, factor V, other coag and growth factors)
- Granules amplify aggregation, promote activation, or cause coagulation cascade
9
Q
Platelet plug formation 3
A
- Platelet aggregation is caused by the binding of platelets to each other through the IIb/IIIa receptor
- IIb/IIIa binds either vWF (multimeric, best in high sheer arteries) or fibrinogen (dimeric, best in low sheer veins)
- Aggregation is amplified by generation of thromboxane A2 (TxA2)
- TxA2 is an eicosanoid generated from arachidonic acid (taken from membrane by phospholipase) through the COX pathway
- Aspirin irreversibly inhibits the COX nz, while other NSAIDs reversibly inhibit it
- TxA2 will cause other platelets to express IIb/IIIa, inducing activation and aggregation
- TxA2 is also a potent vasoconstrictor
10
Q
Platelets and coagulation cascade 1
A
- While this is happening, the coagulation cascade is simultaneously being activated
- Basement membrane exposed TF leads to activation of both intrinsic and extrinsic pathways (via factor VIIa activation of factor IX)
- The fibrin polymers that are produced by the coagulation cascade are covalently cross-linked together and to platelets by factor XIII
11
Q
Platelets and coagulation cascade 2
A
- Many of the coagulation reactions require a membrane surface (the ones that require PL)
- These membranes are usually on the platelet surface or endothelial cells
- Platelets that bind to the sub endothelial collagen make a surface for the coagulation cascade
- Other sources of TF: non-cell associated sub endothelial TF, TF on monocytes/macrophages, fibroblasts, glial cels, blood-borne micro-vesicles (released from monocytes/macrophages in BM)
12
Q
Platelets and coagulation cascade 3
A
- TF can be upregulated on the surface of cells (endothelium and monos/macs) by a number of factors: cytokines, endotoxins, GFs (VEGF)
- P-selectin on activated platelets binds to PSGL-1 on the microvesicles released by monocytes
- The microvesicles also have TF/VII bound to the surface
- When P-selectin and PSGL-1 bind, the microvesicle fuses w/ platelet membrane and the TF/VII becomes displayed on the platelet, facilitating the coagulation cascade by more TF expression
13
Q
Disorders of excessive TF expression
A
- Primarily can result in disseminated intravascular coagulation (DIC) or localized thrombosis
- Cytokines released during infection induces expression of TF on endothelial cells and monocytes/macrophages
- TF can also be released from cells due to trauma or expressed on tumor cells, both can result in DIC/thrombosis
- The ability of factor VII to bind to a specific site of injury (where TF is released/expressed) and selectively enhance its function, resulting in the coagulation cascade, is the underlying theme of hemostatic plug formation
14
Q
Tissue factor pathway inhibitor (TPFI)
A
- TF/VIIa complex has the ability to activate both IX and X
- However, when it binds to X the complex is inactivated by TFPI
- This requires that hemostasis must proceed through IX and VIII tenase complex (allowing for more control of hemostasis)
15
Q
Activation of IXa and VIIIa (tenase complex)
A
- Factor IX bound to its receptor on endothelial cells and/or platelets gets activated by the TF/VIIa complex
- IXa can now bind to its cofactor VIIIa (which is already complexed w/ vWF) to form the tenase complex
- The tenase complex activates X to Xa (process requires negatively charged surface, such as an activated platelet membrane or a monocyte membrane)
16
Q
Disorders of IX, VIII, and vWF
A
- Functional or quantitative deficiency of IX is hemophilia B (severe hemorrhagic disorder)
- Functional or quantitative deficiency of VIII is hemophilia A (hemorrhagic disorder)
- A is more common but less severe than B
- VIII must be bound to vWF or else it is cleared, thus if vWF level increases VIII levels also increase
- vWF is acute phase reactant (APR) protein, meaning its levels increase w/ inflammation
- This is one reason why chronic inflammatory diseases are associated w/ high chance of thrombosis and stroke (more VIII)
- Quantitative or functional deficiency of vWF is vonWillebrand’s disease, and has similar characteristics w/ hemophilia A (since lower vWF leads to lower VIII)
17
Q
Vitamin K dependent proteins
A
- The vit K dependent proteins are II, VII, IX, X, and protein C/S
- Only C/S are anticoagulants
- Vit K used to make a g-carboxyglutamic acid on these proteins
- This altered residue is essential for Ca binding and thus conformation change to induce activation
- Therefore w/o vit K the proteins cannot bind Ca and cannot be activated
- The conformation change induced by Ca leads to exposure of a consensus sequence in the amino terminus, required for factor binding to its receptors
18
Q
Disorders of vit K metabolism
A
- Vit K deficiency is rare in adults
- Only occurs in those who are very Ill, not eating and receiving antibios, or in those with impaired fat absorption (such as biliary obstruciton- vit K is fat soluble)
- Vit K deficiency can occur in premature newborns resulting in hemorrhagic disease of the newborn
- Lack of vit K leads to inability of II, VII, IX, and X to undergo activation, thus predisposes a person to bleeding
- Warfarin (coumadin) inhibits the action of one of the nzs needed in the g-carboxyglutamate rxn (warfarin is vit K antagonist)
- Therefore warfarin is an anticoagulant by blocking the function of factors II, VII, IX, and X
- Overdose of warfarin can lead to excessive bleeding
19
Q
Factor X, V and the prothrombinase complex
A
- Activated Xa (by the tenase complex) requires the cofactor Va (activated by trace thrombin) to form the Xa/Va prothrombinase complex
- Coag nzs are essentially inactive unless they are bound to their cofactors and receptors (such as Va, Ca, PL)
- When all of the factors are assembled (Xa/Va/Ca/PL) the prothrombinase complex activates prothrombin to thrombin to begin fibrin production and polymerization
20
Q
Disorders of factor X and V
A
- Disorders of these factors are very rare and generally result in severe hemorrhagic disorders
- A mutation in V (called V Leiden) exists in some people where an Arg is substituted for a Glu
- This mutation renders factor V resistant to inactivation by protein C
- This results in an inherited thrombotic disorder
21
Q
Protein C/S regulation of coagulation
A
- Factor VIIIa is required for activation of the tenase complex, and factor Va is required for activation of the prothrombinase complex
- A proteins complex (activated protein C/S: APC) is responsible for proteolysis of both factor VIIIa and Va
- APC is made from the complex of thrombin and thrombomodulin on the endothelial cell surface
- This complex binds to protein C (which is bound to its endothelial cell receptor, EPRC) and activates it to APC
- APC binds to platelets and endothelial cells in complex w/ protein S to destroy endothelial and platelet bound VIIIa and Va
- Both protein C and S are vit K dependent proteins
22
Q
Disorders of protein C and S
A
- Quantitative or functional deficiencies of protein C or S can result in a congenital thrombotic disorder due to inability to inhibit coagulation
- Children born w/ complete C/S deficiency have the fatal disease purpura fulminans
- Patients w/ C/S disorders who are placed on coumadin and do not receive concomitant heparin can become transiently hyper coagulable and develop cutaneous thrombosis (coumadin necrosis)
23
Q
Prothrombin and thrombin
A
- Thrombin has many functions, it is an enzyme and a mitogen (induces mitosis)
- A lack of thrombin is not compatible w/ life
- Thrombin can initiate hemostatic plug, initiate tissue repair, and maintains vascular fluidity by limiting hemostatic plug extension
- Thrombin can amplify itself by activating XI, VIII, and V
- It also is them most potent activator of platelets, causing them to aggregate and form a surface for coagulation
- These activated platelets express IIb/IIIa to bind vWF/fibrinogen (leading to aggregation) and also express receptors for factors IX and X
24
Q
Thrombin enzymatic function
A
- Proteolytically cleaves fibrinogen into fibrin, releasing fibrinopeptides that polymerize by hydrogen bonding
- This is the primary (or unstable) fibrin clot
- The clot is stabilized by cross linking of fibrin polymers to each other and platelets by factor XIII
- Disorders of quantity of fibrinogen result in bleeding disorders (usually after trauma)
- Disorders of quality of fibrinogen result in either bleeding or thrombotic disorders
25
Q
Limiting coagulation
A
- Mechanisms associated w/ thrombin inhibition are concentrated on surface of endothelial cells
- Antithrombin is the most important, it is only active when associated w/ heparin-like molecules (glycosaminoglycans, GAGs)
- GAGs are present on the endothelium and subendothelium, thus active antithrombin is present at site of thrombin generation
- Quantitative or qualitative deficiencies in antithrombin results in inherited thrombotic disorder
26
Q
Factor XIII
A
- XIII is activated by thrombin, and covalently cross-links fibrin polymers together
- It also covalently links fibrin to platelet fibrinogen/surface proteins
- This forms the stable clot
- Disorders of XIII: quantitative or functional disorders result in delayed bleeding, that is a lesion will stop bleeding but then rebelled due to instability of the clot (wounds slow to heal, women have difficulty maintaining pregnancy)
27
Q
Thrombin and vascular repair
A
- Thrombin stimulates platelets to release platelet derived growth factor (PDGF), and platelet derive endothelial growth factor (PDEGF)
- Thrombin induces monocyte/macrophage chemotaxis and activates them
- These activated cells release many nzs and factors for tissue remodeling
- Thrombin is a mitogen for smooth muscle cells
28
Q
Thrombin and vascular fluidity 1
A
- Thrombin also maintains fluidity by limiting fibrin amount
- Thrombin induces endothelial cells to release protaglandins (inhibit platelet aggregation) and nitric oxide (NO; to dilate vessels and inhibit platelet aggregation)
- Thrombin induces release of tPA and uPA (plasminogen activators), which induce the formation of plasmin and thus fibrin digestion
- Thrombin induces endothelial cells and monos/macs to express uPA receptor (uPAR) increasing plasmin generation
- Thrombin induces expression of tPA receptor (annexin) and produces fibrin (which tPA binds to), both of these can activate plasmin when tPA is bound
29
Q
Thrombin and vascular fluidity 2
A
- Thrombin bound to thrombomodulin is directly inhibited, but this complex also can activate protein C
- Activated protein C degrades the cofactors of coagulation cascade, VIIIa and Va
- APC also enhances fibrinolysis by inactivating plasminogen activating inhibitor (PAI1, which degrades tPA and uPA), leading to an increase in plasmin
- Heparin cofactor II (HC) directly inhibits thrombin
- Thrombin induces platelet mediated clot retraction
30
Q
Intrinsic pathway details
A
- Severe factor XI deficiency can cause mild-moderate bleeding, its not required for basal maintenance of coagulation but plays a role when system is stressed (trauma)
- XIa can activate IX w/o cofactors, and XI is activated by thrombin, not XIIa
- XI provides an amplification loop when higher levels of thrombin are needed
31
Q
Central role of thrombin
A
- Activates factors XI, VIII, V (procoagulation)
- Induces tPA release and receptor/cofactor binding, as well as APC activation (anticoagulation)
- PAR activation (activates platelets, regulates smooth muscle cells and vascular tone), VEGF release, metaloprotease activation (angiogenesis)
- Induction of IL6 and IL8, promotes chemotaxis and activation of monocytes (inflammation)
