Flashcards in EXAM #1: HEMOSTASIS Deck (34)
Prevention of blood loss at a site of injury.
Describe how platelets adhere to the extracellular matrix exposed at a site of vascular injury.
- Vascular damage exposed ECM
- Platelets adhere to the ECM and activate
Specifically, TF initiates the coagulation cascade.
What is the role of von Willebrand factor and glycoprotein Ib (GPIb)?
- vWF is produced by endothelial cells
- Vascular damage exposes the ECM and vWF
- Platelets contain GPIb that binds vFW
This links the platelet to the damaged ECM. Note that platelets also contain - Integrin a1B1 binds collagen.
What is the importance of von Willebrand factor in the normal function of factor VIII?
vWF also binds Factor VIII, which prolongs the half-life of VIII
Describe the morphological changes that occur upon platelet activation.
- vWF and GP1b binding triggers Ca++ signaling in the platelet
- Morphological change causes the platelet to change from a disc shape to a spherical shape with filopodia
What sorts of compound are secreted by activated platelets?
- Factor V
Vasoconstriction and further platelet activation
Explain the process of platelet aggregation, including the roles of aggregation signals, GpIIb-GpIIa, and fibrinogen
- ADP causes a change in conformation of GpIIb/IIIa receptor, which allows it to bind FIBRINOGEN
- Fibrinogen is the glue that binds platelets together i.e. allows platelets to aggregate
*****At this point the platelt "plug" has been formed.
Outline the pathophysiology of von Willebrand disease?
This is the most common platelet defective clotting disorder
- vWF functions in initial platelet binding and extends the half-life of Factor VIII
- Causes excessive bleeding, bleeding from dental treatment...etc.
What is Bernar-Soulier Syndrome? How is it treated?
Inherited platelet disorder hallmarked by:
- GIANT platelets that fail to aggregate
****Generally, there is a defect in the vWF and GpIb interaction; the precise mechanism is unclear.
Treatment includes limiting bleeding risk and platelet transfusion when bleeding is inevitable.
What is Glanzmann thrombasthenia?
This is an inherited bleeding disorder caused by:
- Defect in GpIIb or GpIIIa
- Platelets fail to aggregate b/c they cannot bind fibrinogen i.e. the "platelet glue"
Therapy aims to prevent bleeding and transfusion.
What is the importance of vitamin K in the coagulation cascade? What enzyme is Vitamin K an important cofactor for?
- Vitamin K activates Prothrombin (II), VII, IX, X and Proteins S and C
- Specifically, Vitamin K is a cofactor for Carboxylase, which converts glutamate to y-carboxyglutamate
- y-carboxyglutamate chelates Ca++ and traffics bound Ca++ to negatively charged membrane lipids
Thus, Vitamin K is required to target these clotting factors to the site of injury.
How does warfarin function in the prevention of blood clotting?
- Warfarin inhibits Vitamin K dependent activation of clotting factors
- Specifically, Warfarin inhibits the activation of Vitamin K epoxide to Vitamin K via Vitamin K epoxide reductase
Outline activation of thrombin via the extrinsic pathway.
- Damage to the endothelium exposes Tissue factor
- In the presence of Ca++ Factor VII binds TF and is activated
- Factor X associated with TF/VIIa to form EXTRINSIC XASE COMPLEX
- Factor VII cleaves Factor X to Xa
Include the roles of the various components of the extrinsic Xase complex.
- Generation of Xa via VIIa cleaving X
- Slow cleavage of prothrombin to thrombin
Note that Thrombin activates Factor V.
Describe how the prothrombinase complex generates active thrombin
Prothrombinase complex is:
- Factor Xa
This complex RAPIDLY cleaves prothrombin to thrombin
What are the roles of thrombin in generating the insoluble fibrin clot?
- Cleaves fibrinogen to fibrin that forms the insoluble fibrin clot
What is the role of thrombin in and sustaining and accelerating the extrinsic and intrinsic coagulation pathways?
- Activates Factor V and VII of the extrinsic pathway
- Activates Factor VIII of the intrinsic pathway
Outline the activation of thrombin via the intrinsic pathway as it is thought to occur in the body.
1) Kallikrein= XII to XIIa
4) IXa activates Factor X
5) Factor Xa generates low levels of thrombin
6) Thrombin activates Factor VIII
Factor VIIIa and Ca++ form the intrinsic Xase complex with Factor X that rapidly activates Factor X to Xa. Xa rapidly activates thrombin.
Outline the pathophysiology of Hemophilia A.
- Most common
- Factor VIII deficiency
Outline the pathophysiology of Hemophilia B.
- Less common
- Factor IX deficiency
How does thrombin, via thrombomodulin, protein C, and protein S function in anticoagulation?
1) Thrombin + thrombomodulin activates Protein C
2) Protein C and Protein S complex
3) Protein C/S complex inhibits Factor VIIIa and Va to block clotting
Thus, thrombomodulin changes the specificity of Thrombin
What is ‘factor V Leiden’ deficiency?
- Arginine in amino acid position 506 is changed swapped with glutamine in Factor V
- This mutation makes Factor V resistant to cleavage by protein C
What are the consequences for patients with this form of factor V?
Increases the risk for thrombosis
What is meant by the term ‘serpins’ ?
This is a family of serine protease inhibitors that INHIBIT the activated forms of coagulation factors
- ANTITHROMBIN III is the most important of the serpins
What is ‘tissue factor pathway inhibitor (TFPI)’?
- Protein produce by endothelial cells
- Inhibits VIIa to block the extrinsic pathway
- Also inhibits Factor Xa
How is fibrinolysis achieved?
- Fibrin is degraded by the serine protease plasmin
- Plasmin circulates as the inactivated plasminogen
- Plasminogen is activated to plasmin by TISSUE PLASMINOGEN ACTIVATOR i.e. t-PA
What is streptokinase and how does it function?
This is an exogenous activator of plasminogen produced by B-hemolytic streptococci
How is vFW treated?
Desmopressin, which induces the release of vWF and Factor VIII from storage sites in the endothelium
Outline the entire coagulation cascade.