Haemostasis Flashcards
What is included in Primary Haemostasis?
Forms unstable platelet plug
Platelet Adhesion to endothelium
Platelet Aggregation
What is Secondary Haemostasis?
Stabilisation of the plug with fibrin
Blood coagulation
What is Fibrinolysis?
Dissolution of clot and vessel repair
What are platelets? What are they derived from?
Platelets are discoid, non nucleated, granule containing cells
Derived from myeloid stem cells
Formed in bone marrow by fragmentation of megakaryocyte cytoplasm
How can Platelets stick to damaged endothelial cells directly and indirectly?
Directly to collagen via the platelet GPIa receptor
Indirectly via von Willebrand factor which wil bind to platelet GPIb receptor.
What is the platelet release reaction?
What substances are released?
Adhesion of platelets causes their activation
Once activated release alpha-granules and dense granules through membrane invagination.
Released substances : ADP, fibrinogen, vWF
How is Thromboxane A2 synthesised?
Arachidonic Acid –> Cyclic Endoperoxides –> (Activated platelets use this to produce ) Thromboxane Synthetase –> Thromboxane A2
What are the roles of Thromboxane A2?
Platelet aggregation
Vasoconstriction
How is a positive feedback loop achieved to increase platelet aggregation?
Granular release of ADP from platelets and generation of thromboxane A2
ADP binds to P2Y12
thromboxane binds to thromboxane A2 receptor
What role does Fibrinogen play in the primary haemostasis positive feedback loop?
Platelet activation causes a change in the GPIIb/IIIa receptor so there are binding sites for fibrinogen
Fibrinogen binding to this receptor further activates platelets
How is the positive feedback loop controlled?
Counterbalanced by flow of blood and release of prostacyclin PGI2 from endothelial cells which acts as a vasodilator suppressing platelet activation
What is Aspirin used for, what does it do to achieve these results?
Antiplatelet drug
inhibits thromboxane A2 production by blocking action of COX
effects last for 7 days ( as platelets will be replaced in that time)
What is Clopidogrel used for, what does it do to achieve these results?
Antiplatelet drug
irreversibly blocks ADP receptor P2Y12 on platelet membrane
effects last for 7 days
What are the characteristics of VWF and what other role does it play apart from adhesion?
A glycoprotein synthesised by endothelial cells and megakaryocytes.
Circulate as multimers
VWF is a specific carrier for factor VIII ( FVIII) (8)
Where are clotting factors synthesised? Are there any exceptions?
In the liver
Except for VIII (8) and VWF which are made by endothelial cells
Which clotting factors are dependent on Vitamin K and why?
Factors (2), (7), (9), (10)
VK is needed for carboxylation of their glutamic acid residues, this is essential for their function
How is an inactive zymogen turned into an active clotting factor?
the proezyme (inactive zymogen) has its peptide bones split to expose active enzyme sites.
Which clotting factors act as co-factors?
Factors 5 and 8
What role to Calcium ions play?
Binding of activated clotting factors to phospholipid surfaces of platelets which helps accelerate reactions.
The trigger to initiate coagulation at the site of injury is the (a) exposed on the surface of endothelial cells and (b) and on most extravascular cells.
Where is (a) usually located?
a - tissue factor (TF)
b - leukocytes
TF is usually at sites not exposed to blood, blood only encounters TF at injury and so it can act as a stimulus for coagulation.
The binding of TF to factor (a) leads to the activation of factor (b) to (c) and factor (d) to (e). This leads to activation of (f) to generate a small amount of thrombin (also known as g)
a - 7a b - 9 c - 9a d - 10 e - 10a f - prothrombin (2) g- 2a
What does Thrombin do?
Mediated activation of Co-factors (5/8), the zymogen factor 11 and platelets
What occurs in the Amplification phase?
Factor 11 converts more 9 to 9a which with factor 8a amplifies conversion of factor 10 to 10a and hence rapid burst of thrombin
What occurs in the Propagation Phase?
Cleaves fibrinogen so it is not soluble and forms insoluble fibrin clot
How does protein C work with protein S as an anticoagulants?
Thrombin binds to thrombomodulin on endothelial cell surface which leads to protein C activation = APC.
APC then inactivates factors 5a and 8a in presence of protein S
How is Thrombin and factor 10a inactivated?
Antithrombin.
Its action is potentiated by Heparin which occurs when antithrombin binds to endothelial cell heparins.
What intravenous anticoagulant can be given?
What does it do?
Heparin - it is a mixture of glycosaminylglycan chains
potentiates antithrombin inactivating thrombin and 10a
( inactivation of thrombin requires longer chains of heparin chains which can wrap around both antithrombin and thrombin.)
What is Warfarin and what does it do?
Derived from coumarin, it is a VK antagonist and interfares with protein carboxylation. So it an anti-coagulant.
Reduced factors produced by liver
How is Warfarin given?
As an oral tablet, its effects need to be monitored by regular blood testing.
It doesn’t inhibit existing factors but reduces synthesis, takes several days to see effect
What other anticoagulants are available?
Direct oral anticoagulants (DOACs)
directly inhibit thrombin or factor 10a, without involvement of antithrombin
do not require monitoring
What is the principle fibrinolytic enzyme?
Plasmin which circulates in a inactive form plasminogen
How does t-PA activate plasminogen? ( tissue plasminogen activator )
Both t-PA and plasminogen need to be bound to lysine residues on fibrin. Then it activates it into plasmin
What other protein componants can Plasmin break down?
How is Plasmin inhibited?
Fibrinogen, clotting factors 5a and 8a
antiplasmin circulates blood
What can be given to px presenting with Ischaemic stroke?
also for pulmonary emboli
Thrombolytic agents such as recombinant t-PA to create plasmin and remove clots.
Time dependent therapy, preferably given within an hour of symptom onset
What is Tranexamic acid?
synthetic derivative of lysine that acts as an antifibrinolytic drug
Used to treat trauma, surgical px and bleeding disorders.
How does Tranexamic acid work?
Binds to plasminogen, preventing it from binding to lysine residues as it acts as a competitive inhibitor.
What does the intrinsic and extrinsic system compromise of respectively?
Factors 12, 11, 10 , 9, 8 and 5) found in plasma
Factors 7, 10, 5 and TF
Why was the extrinsic-intrinsic model disregarded?
Used to believe both pathways happened in parallel.
The intrinsic pathway began with the activation of factor 12.
Although now we know people who don’t have factor 12 do not have bleeding problems
What does the Prothrombin time measure? And why may it be prolonged?
The integrity of the extrinsic pathway
If there is a reduction in activity of factors 7, 10, 5, 2 (prothrombin) or fibrinogen
PT method
Blood is collected in a bottle containing (a) preventing clotting.
Sample is spun to produce platelet poor plasma.
A source of both (b) and (c) - such as (d) is added together with calcium to start reaction in plasma.
Time taken to (e) is recorded
a - sodium citrate
b - TF
c - phospholipid
d - recombinant thromboplastin
e - clot
How are the different thromboplastin reagents corrected across different labs if they are from various sources?
Results are expressed as an international normalised ratio. INR which means the INR should be the same for the same sample across different labs or reagents
What does APTT measure?
Why might Activated partial thromboplastin time be prolonged?
Integrity of the intrinsic system
Reduction in a single or multiple clotting factors. ( if many reduced there may also be a reduction in PT time)
What PXs is a longer APTT but normal PT time seen?
Haemophilia A - factor 8 deficiency
Haemophilia B - factor 9 deficiency
Factor 11 deficiency
Factor 12 deficiency
(note factor 12 is not in cell-based model invivo, just comes up in intrinsic model)
How is APTT measured?
Contact activation of factor 12 using a contact activator such as silica or kaolin.
Added with phospholipid to citrated plasma followed by calcium.
Time taken to clot measured.
What three things may cause bleeding?
Reduction in platelet number or function ( platelet plug )
Reduction in coagulation factors ( fibrin clot )
Increased fibrinolysis
What may cause thromobycytopenia?
Number and function?
Platelet production issues : Drugs, viruses, bone marrow infiltration, megaloblastic anaemia, hereditary thrombocytopenia
Shortened survival due to DIC
Increased splenic pooling
Reduced function often due to antiplatelet drugs, inherited causes
What are congenital causes of reduced coagulation factors?
From birth
E.g.
Von Willebrand disease, most common bleeding disorder
Haemophilia A, B (X-linked factor 8 and 9 deficiency respectively)
What are the acquired causes of reduced coagulation factors?
Liver disease
Anticoagulant drugs
Disseminated intravascular coagulation - DIC
(uncontrolled activation of coagulation with activation of fibronlytic system)
What does DIC cause?
TF expression increased leads to Thrombin creation and destruction. Uses up lots of platelets - thrombocytopenia
Widespread Thrombi can cause shearing of r.b.c so schistocytes may be seen on blood film
Clotting factors and fibrinogen depleted leading to sever and life threatening bleeding.
High levels of fibrin degradation products
What are the causes of DIC?
Bacterial sepsis
Advanced cancer
Obstetric emergencies
What is Thrombosis and What is the Virchow’s triad?
Formation of blood clot within intact blood vessel
Three contributory factors:
Blood - Dominant in venous thrombosis
Vessel wall - Dominant in arterial thrombosis
Blood flow - contributes to both
What changes in blood can increase risk of venous thrombosis? 3
Reduced levels of anticoagulant proteins
Reduced levels of fibrinolytic activity e.g. pregnancy when plasminogen inhibited by Placenta PAI-1
Increased levels of Clotting factors or platelets
Levels of factor 8 increase during pregnancy
Factor 5 can increase by point mutation in its gene ( factor V Leiden) making it more resistant to inactivation by protein C
Increased platelets in myeloproliferative disorders
