Haemostasis Flashcards
What are the steps involved in normal haemostasis?
Injury
- Vessel constriction
- Formation of unstable platelet plug
a) Platelet adhesion
b) Platelet aggregation - Stabilisation of plug with fibrin
a) Blood coagulation - Dissolution of clot and vessel repair
a) Fibrinolysis
Why does the unstable platelet plug need to be stabilised?
Alone, unstable platelet plug quickly breaks down
What are the functions of endothelial cells in haemostasis?
Maintain barrier between blood and procoagulant subendothelial structures
Synthesise prostacyclin, thrombomodulin, vWF, plasminogen activators
How does a platelet plug form?
- Endothelial monolayer is damaged
- Subendothelial structures exposed
- Platelets bind to collagen in 2 ways:
a) vWF binds to collagen at site of damage and unfolds so that it can capture platelet via GPIbRs
b) Platelets bind directly to collagen via GPIaRs - Receptors signal inside cell to release ADP from storage granules and to synthesise and release thromboxane
- ADP and thromboxane bind to Rs on platelet surface + activate platelets
- Platelets release storage granules
- Once platelet is activated, GPIIb/IIIaRs become available
- Rs can bind fibrinogen - helps platelets clump together = aggregation
- Platelets aggregate and form haemostatic plug
How are platelets activated?
By ADP and thromboxane released from platelet
OR
by thrombin (generated in coagulation)
What does platelet activation lead to?
Prostacyclin metabolism
Explain prostacyclin metabolism
Platelet activation
Occurs in endothelial cells and platelets
1. Phospholipase mobilises the membrane phospholipid and converts it to arachidonic acid
2. Arachidonic acid –> endoperoxides (PGG2, PGH2) by COX
3. Endoperoxides to:
a) prostacyclin (PGI2) by prostacyclin synthetase in endothelial cells
b) thromboxane A2 by thromboxane synthetase in platelets
How does prostacyclin affect platelet function?
Potent inhibitor of platelet function
How do endoperoxides and thromboxane affect haemostasis?
Potent inducers of platelet aggregation
What lab tests are used to investigate coagulation?
Activated partial thromboplastin time (APTT)
Prothrombin time (PT)
Thrombin clotting time (TCT)
How can coagulation test results contribute to a clinical diagnosis?
APTT and PT used together for screening for causes of bleeding disorders
APTT used to monitor heparin therapy for thrombosis
PT used to monitor warfarin treatment in thrombosis
How does warfarin work?
Normal coagulation:
Nascent clotting factors in liver have glutamic acid cluster
Recognised by enzyme and converted in post-translational modification in presence of vitamin K to gamma-carboxyglutamic acid
Once extra carboxyl group added, calcium facilitates binding of gamma-carboxyglutamic acid to activated platelet membrane phospolipid
Warfarin inhibits vitamin K epoxide reductase - enzyme for gamma carboxylation
Inhibits localisation of CFs on platelet surface
Tf reduces thrombin production
How does heparin work?
Heparin potentiates action of plasma inhibitor antithrombin
Makes antithrombin work more efficiently at directly inhibiting FXa and thrombin
Heparin binds to antithrombin
Changes position of reactive loop
What are the indications for warfarin?
Long term anticoagulation following venous thrombosis
Treatment of AF
What are the indications for heparin?
Immediate anticoagulation in venous thrombosis and pulmonary embolism
How do you measure the effects of warfarin and heparin in the lab?
APTT used to monitor heparin therapy in thrombosis
PT used to monitor warfarin treatment in thrombosis
What are the different types of antiplatelet agents?
- ADP receptor antagonists - inhibit platelet activation
- COX-1 antagonist - inhibit PGI2 metabolism
- (GPIIb/IIIa antagonists)
What are the indications for antiplatelet therapy?
CVD
During interventions, e.g. angioplasty, stents
Why is heparin used over warfarin for immediate anticoagulation?
Warfarin takes more time to build up in the circulation and become effective
How does antithrombin work?
Circulates in high conc in blood
Directly inhibits FXa and thrombin
Reactive loop irreversibly blocks active site of coagulation factors
Antithrombin acts as a scavenger that stops inappropriate CF action
How does the action of heparin differ when it’s inhibiting FXa vs. thrombin?
Heparin binds to thrombin
Changes position of reactive loop
Accelerates inhibition
FXa: SHORT section of heparin chain is sufficient to make antithrombin block FXa more quickly
Thrombin: LARGE heparin chain needed
How do the 2 forms of heparin differ?
Low molecular weight heparin = favours FXa inhibition
Standard/unfractionated heparin = inhibits FXa or thrombin
What is APTT used for?
Activated partial thromboplastin time
Detects abnormalities in intrinsic and common pathways
Initiates coagulation through FXII activation
Measures clotting time
Used to monitor heparin therapy for thrombosis
Used with PT for screening for causes of bleeding disorders
What is PT used for?
TF added to trigger extrinsic pathway
Detects abnormalities in extrinsic and common pathways
Used to monitor warfarin treatment in thrombosis
Used with APTT for screening for causes of bleeding disorders
