Haem - Haemostasis Flashcards
What is haemostasis?
Blood clot
Steps in haemostasis
- Vasoconstriction
- Primary haemostasis
- Secondary haemostasis
- Fibrinolysis
What is primary haemostasis?
Formation of unstable platelet plug (Platelet adhesion/aggregation)
What is secondary haemostasis?
Formation of stable fibrin clot
What is fibrinolysis?
Dissolution of a clot
Important balances in haemostasis
- Coagulation (state change)
- Thrombosis (Limit area)
- Fibrinolysis (breakdown clot, healing)
Where are platelets formed?
In the bone marrow, derived from myeloid stem cells, it is a fragmentation of megakaryocyte cytoplasm
What is von Willebrand factor VWF?
Factor in mediating platelet adhesion, also promotes platelet-platelet aggregation
What happens in primary haemostasis?
- Platelet stick to damaged endothelium (…)
- Adhesion activates platelets (…)
- Platelet aggregates
How does platelet stick to endothelium?
Either directly to collagen via GP1a receptor
OR
indirectly to the VWF factor with binds to GP1b receptor
How does adhesion activate platelet?
- Membrane invaginated to release content of granules
- ADP, fibrinogen, VWF - Changes shape (round w/ spicules) - increase platelet-platelet interaction
- Produces prostaglandin thromboxane A2
What are the granules of platelet?
- alpha-granule
- dense granule
What does thromboxane A2 do?
- Promotes platelet aggregation
2. Vasoconstrictor (imp. in injury & inflammation)
What signals further activation and aggregation in primary haemostasis?
Combo of ADP & thromboxane, giving positive feedback
How does platelet further aggregate?
By binding to ADP (P2Y12) and thromboxane A2 receptors
How does ADP + ThA2 cause further activation?
Conformational change in GPIIb/IIIa receptor, provide binding site - fibrinogen- to link platelet to form plug
What chemical counter balances and stops platelet aggregation?
Prostacyclin (GPI 2) - powerful vasodilator & suppress platelet adhesion
What is secondary haemostasis?
Clotting involving the coagulation cascade
[formation of fibrin]
What HAPPENS in secondary haemostasis?
Generation of thrombin –> cleaves fibrinogen –> form fibrin clot –> stabilise plug
What is factor II?
Prothrombin
What does the synthesis of clotting factors depend on?
Vitamin K (for carboxylation of glutamic acid residue) essential for clot function
What do clotting factors work on?
Exposed platelet phospholipid surfaces (help localise & accelerate reactions)
Where are clotting factors synthesised?
Liver (except for factor VIII & VWF - endothelial cell)
What initiates coagulation?
Tissue factor - not normally exposed to blood, only injury
How does tissue factor initiate coagulation?
It binds to factor VIIa
- activates factor IX –> XIa
- activates factor X –> Xa
Activates prothrombin (Factor II)
What are the 3 phases of secondary haemostasis?
- Initiation
- Amplification
- Propagation
What does activating prothrombin do?
Generate thrombin (factor IIa)
What does thrombin do? (propagation)
Cleaves soluble fibrinogen to insoluble fibrin
What does small amount of thrombin mediate?
Activation of cofactor V & VIII, zymogen (factor XI) & platelets
What is the amplication phase?
Zymogen converts more FIX –> FIXa
FIIIA converts more FX –> FXa
Therefore, more thrombin (FIIa) converted from Prothrombin (FII)
What ion is important in binding activated clotting factors to phospholipid surfaces?
Calcium ion
What does Zymogen (FXI) turn?
Activate clotting co-factors
FV –> FXI
FVIII –> FVIIIa
(by splitting of peptide bonds & expose enzyme sites)
What happens during fibrinolysis?
Tissue plasminogen activator (t-Pa) activates plasmin by binding to lysine residue on fibrin
Fibrin is broken down into Fibrin-degradation products (FDPs)
What was plasmin before it was activated?
Inactive Plasminogen
What do anti-platelet drugs do?
Prevention and treatment of cardiovascular/cerebrovascular disease
How does anti-platelet drugs work?
- Inhibit thromboxane A2 production
- Block ADP receptor
Hence decrease platelet aggregation, and prevent formation of thrombus
What are some examples of anti-platelet drugs?
Aspirin, Clopidogrel
How long does anti-platelet drug last?
Until new platelets are produced (around 7 days)
What do thrombolytic therapy/agents do?
eg Recombinant t-Pa intravenously injected to generate plasmin to breakdown clots
What is the risk w/ thrombolytic therapy?
Bleeding
What are thrombolytic therapies used for? What replaced them?
Ischaemic stroke
Myocardial infarction
Life-threatening pulmonary emboli
(Stents, angioplasty)
How does antifibrinolytic drug (Tranexamic acid) work?
Competitive inhibition - derivative of lysine
- binds to plasminogen
- prevent bind to lysine residue of fibrin
- prevent activation of plasmin
What are antifibrinolytic drugs used for?
Treat bleeding in:
- Trauma surgical patients
- Inherited bleeding disorders
What happens when the balance leans towards fibrinolytic factors and anticoagulant proteins?
Bleeding
- reduce platelet function
- reduce coagulator factors
- increase fibrinolysis
What happens when the balance leans towards coagulation factors and platelets?
Thrombosis - clot in vessel
- reduce anticoagulant proteins
- reduce fibrinolysis
- increase clotting factor/platelet
Coagulation inhibitory mechanism:
- Thrombin binds thrombomodulin
- activates Protein C –> “activated protein C (APC)” - APC activates FVa &FVIIIa in presence of cofactor Protein S
- Thrombin & FXa inactivated by Antithrombin
How does heparin increase action of antithrombin?
Antithrombin binding to endothelial cell-associated heparin
What use is the anticoagulant pathway?
To confine coagulation at site of injury
What are the main substrates for anticoagulation?
Protein C
Protein S
Antithrombin
What are some examples of anticoagulant drugs? (prevent thrombosis)
Heparin, Warfarin, Direct Oral Anticoagulants (DOACs)
How does Heparin drug work?
Potentiate action of antithrombin –> inactivate FXa and thrombin
How does Warfarin drug work and monitored?
Oral tablet
- Interferes protein carboxylation / antagonizes Vitamin K
- Reduces synthesis of vit K-dependent clotting factors (thrombin, VII, IX, X) produced by liver
Monitored by regular blood tests
How does Direct oral anticoagulants drug work and monitored?
Directly inhibits either thrombin/FXa
No monitor
What are the tests of coagulation?
- PT (Prothrombin time)
- APTT (Activated partial thromboplastin time)
What does prothrombin time measure?
Time to clot following extrinsic pathway
What does aPTT measure?
Time to clot following intrinsic pathway
How to do a PT test?
- Blood collected in bottle w/ sodium citrate (chelates Calcium to initially prevent clot)
- Sample spun - produce platelet-poor plasma
- Add Tissue factor & phospholipid + calcium
Time…
What does a ‘prolonged PT’ tell you?
Reduction in activity of FVII, X, II prothrombin, fibrinogen
How to do an aPTT test?
Add contact activator, phospholipid, citrated plasma and calcium
Time…
What does a ‘prolonged aPTT’ tell you?
Reduced in one/more clotting factor
What does a ‘prolonged aPTT’ and ‘normal PT’ tell you?
Haemophilia A (FVIII deficient)
Haemophilia B (FIX deficient)
Haemophilia C (FXI deficient)
FXII deficient (does not result in bleeding, wrong in model)
What is the combination name of Tissue factor and phospholipid?
Recombinant thromboplastin
What does INR stand for?
International normalized ratio
