7. Haemostasis Flashcards
Describe the formation of a haemostatic plug?
• Vessel constriction - first-response to vessel injury
• Formation of the unstable platelet plug
- platelet adhesion
- platelet aggregation
• Stabilisation of the plug with fibrin
- blood coagulation system
• If not completely stabilised, some platelet mass embolises
• Dissolution of the clot and vessel repair - fibrinolysis
Which 2 important molecules involved in blood vessel damage repair, are blocked by endothelial cells?
- Collagen - important in the platelet response
* Tissue factor - triggers coagulation
What are the functions of the endothelium with reference to haemostasis?
- Maintain barrier between blood and procoagulant subendothelial structures
- Synthesis of PGI2, thrombomodulin, vWF, plasminogen activators
How does an unstable platelet plug form?
- Collagen exposed in injury
- vWF binds to collagen
- Glycoprotein-1-b receptors on platelets bind to vWF - vWF forms a bridge between platelets and collagen
- Glycoprotein-1-a receptors on platelets bind directly to exposed collagen
- Binding of platelets partially activates the platelet => release of ADP and thromboxane
- Further activation of platelets (positive feedback)
- Glycoprotein IIb/IIIa on platelets are activated - bind to fibrinogen to form a platelet aggregate
How is an unstable clot stabilised?
- Fibrinogen is converted to fibrin
* Thrombin is generated and also activates platelets and the IIb/IIIa complex
How is thromboxane generated within the platelet?
1) Phospholipases activated during platelet activation
2) Act on membrane phospholipids to generate arachidonic acid
3) Cyclo-oxygenase converts arachidonic acid to endoperoxides
4) Thromboxane A2 generated (potent activator of platelets)
How does aspirin work?
Targets COX 1 irreversibly - prevents arachidonic acid => endoperoxides
What are ADP receptor antagonists?
- Drug class of antiplatelet agents
- Prevent platelet activation
- Dampen down platelet response in acute coronary syndrome or those at risk of a clot
- e.g. clopidogrel, prasugrel
What are Gp IIb/IIIa inhibitors?
- Drug class of antiplatelet agents
- Prevent platelet aggregation and thrombus formation
- Used during interventions - angioplasty
- e.g. abciximab, tirofiban, eptifibatide
What is the normal range of platelets in circulation?
150-400 x 10^(9)/l
How is bleeding affected when the platelet count drops below 100 x 10^(9)/l
- No spontaneous bleeding
* Bleeding occurs with trauma
What do we see in the limbs in autoimmune thrombocytopenia and why?
- Extensive bruising of the limbs
* Autoantibodies against the GpIIb/IIIa complex
What are the 2 ways of testing the haemostatic function of platelets?
• Bleeding time
- done when platelet count is normal but bleeding taking place
- tests platelet vessel wall interaction
- can be used in renal disease
• Platelet aggregation
- measures the clumping of the platelets
- used in inherited platelet abnormalities and vW disease
Where are most coagulation proteins made?
Liver
Where is von Willebrand factor made?
Endothelial cells
What is the potent initiator of the clotting cascade?
Tissue factor
Outline the extrinsic pathway of the clotting cascade to the formation of factor 10a
- TF binds to factor 7
- Factor 7 => 7a
- Factor 7a converts factor 10 => 10a
Outline the clotting cascade from factor 10a
- Factor 10a converts prothrombin (II) => thrombin
- Factor 5a can help this process
- Thrombin converts fibrinogen (soluble) => fibrin (2 cleavages) forming a fibrin clot (insoluble)
- Thrombin also converts factor 13 => 13a
- Fibrin clot can be covalently cross-linked by factor 13a to stabilise it - can’t be broken down by shear forces
Outline the intrinsic pathway of the clotting cascade to the formation of factor 10a
Damaged endothelium
• Factor 12 => 12a
• Factor 11 => 11a
(• TF binds to factor 7, => 7a)
• Factor 11a and 7a convert factor 9 => 9a
• Factor 9a converts factor 10 => 10a (using 8a and Pl)
What activates factor 8 and 5?
Traces of thrombin
How does factor 8a work?
- Binds to both factor 9a and 10 on the surface of the platelet
- Pulls them together
- Activation is a proteolytic process
How does factor 5a affect the speed of thrombin generation?
10,000 fold acceleration
How is vitamin K significant in coagulation?
In the liver…
• Glutamic acid (residues in clotting factor molecules) => γ-carboxyglutamic acid
• This binds to the platelet membrane
• Vitamin K mediates this conversion
• Coagulation factors can assemble on the platelet surface
How does warfarin work and what is it used for?
- Anticoagulant
- Inhibits vitamin-K epoxide reductase
- Prevents platelet binding of the clotting factors to the phospholipid membrane
- Complexes not formed - thrombin production inhibited
• Therapeutic agent for long-term anticoagulation
- following venous thrombosis (6 months)
- treatment of AF (lifelong)
How does heparin work and what is it used for?
- Anticoagulant
- Accelerates the action of a natural plasma inhibitor - antithrombin
- Antithrombin inhibits the coagulation proteases: thrombin, 10a, 9a, 11a
- Forms irreversible complexes
• Immediate anticoagulation in venous thrombosis and pulmonary embolism
What are the 3 lab tests for blood coagulation and what do they detect?
• Activated partial thromboplastin time (APTT)
- initiates coagulation through factor 12
- detects abnormalities in intrinsic and common pathways
• Prothrombin time (PT)
- initiates coagulation through TF
- detects abnormalities in extrinsic and common pathways
• Thrombin clotting time (TCT)
- add thrombin
- detects abnormalities in the fibrinogen => fibrin conversion
What are the 3 lab tests for blood coagulation used for?
- Activated partial thromboplastin time (APTT) and prothrombin time (PT) - used together for screening for causes of bleeding disorders
- APTT - monitoring heparin therapy in thrombosis
- PT - monitoring warfarin treatment in thrombosis
What does fibrinolysis involve the activation of?
• Activation of plasminogen by tissue plasminogen activator (tPA)
• Both plasma proteins:
- plasminogen = zymogen
- tPA = proteinase
• When a clot forms, they both assemble on the surface to form => plasmin
When is tissue plasminogen activator used therapeutically?
- Post-myocardial treatment
* Break down clots
How can you detect the fibrin fragments from fibrinolysis?
Assay
What is disseminated intravascular coagulation (DIC) and how does it affect fibrin degradation products?
- Small blood clots develop throughout the blood stream
- Depletes platelets and clotting factors
- Causes excessive bleeding
- Elevation of fibrin degradation products
What is plasmin?
- Powerful proteolytic enzyme
* Breaks down the fibrin clot
What are the 2 mechanisms that prevent the clotting cascade from clotting all of the blood in the body?
- Direct inhibition
* Indirect inhibition
How does direct inhibition work as a coagulation inhibitory mechanism?
- Direct inhibition of the activated coagulation factors
- Inhibitor - antithrombin - circulates in high concentrations
- Broad scale inhibitor of most of the coagulation proteinases e.g. 9a, 10a, thrombin
How does indirect inhibition work as a coagulation inhibitory mechanism?
- Slows down the amount of thrombin (specifically) generated
- Involves the activation of protein C in the protein C anticoagulant pathway
- Activation: thrombin => thrombomodulin => protein C => protein S
- Protein S inhibits 8a and 5a
How are the coagulation inhibitory mechanisms significant as risk factors for thrombosis?
Deficiencies of antithrombin, protein C and protein S are important risk factors
