Haemostasis Flashcards
Normal haemostasis
Blood flows within the vascular system transporting oxygen nutrients and hormonal information around body. Dependant on balancing factors constituting coagulation factors and platelets
Why is balance of blood constituents important
1)coagulation-allow stimulation of blood clotting processes following injury when blood changes from liquid state
2)thrombosis-limit extent of response to the area of injury to prevent excessive or generalized blood clotting
3)fibrinolysis-start process that breaks down the clot as part of the healing process
What is an overall view of the Haemolysis
Halting of blood trauma to blood vessels has 3 processes
1)vasoconstriction of blood vessels
2)primary haemostasis formation of an unstable platelet plug at the site of vessel wall damage
3)secondary haemostasis stabilization of the plug with fibrin
Then fibrinolysis happens
Why is it important to understand homeostatic mechanisms medically
Diagnose and treat bleeding disorders
Monitor drugs used to treat bleeding
Indetify thrombosis risks
Control bleeding in individuals who don’t have an underlying dirosder or blood
Vasoconstriction of primary haemostasis
Nitric oxide and prostacylin (vasodilators) concentrations are less than endothelin (vasoconstriction) concentration reducing the amount of blood being lost
What are platelets
Discoid,non nucleated,granule containing cells derived from myeloid stem cells
Formed in bone marrow by fragmentation of megakaryocyte cytoplasm and circulate for 10 days
Plasma membrane contains glycoproteins
How do platelets react following injury to vessel wall
Stick damaged endothelium either directly to collagen via the platelet GPIa receptor or indirectly by the von willer brand factor which binds to platelet GPIb receptor
Causes activation and change shape from disc to rounded form with spicules to encourage platelet to platelet interaction
What do platelets do after being activated
They release the content of their storage granules
Alpha granules or dense granules
Contents include ADP,fibronegen,VWF
What other chemicals do platelets produce
Prostaglandin thromboxane A2 from arachidonic acid which is derived from the cell membrane
Useful in platelet aggregation but is a vasoconstrictor
Activated platelets also produce calcium
What does the release of ADO and generation of thromboxane A2 do
ADP binds to p2y12 receptor and thromboxane binds to thromboxane A2 receptor which has a positive feedback effect
How does fibrinogen bind to platelets
Platelet activation causes a conformational change in the GPIIb/IIIa receptor (inside out), which provide binding sites for fibrinogen
This binding causes outside-in signalling which further activates the platelets
Fibrinogen has a key role in linking platelets together to form the platelet plug
How is inappropriate platelet aggregation avoided
Active flow of blood and release of prostacylin (PG12)
Is a powerful vasodilator
Released by endothelial cells
What does aspirin do an an antiplatelet drug
Inhibits the production of thromboxane A2 by irreversibly blocking the action of cyclo-oxygenase (COX), resulting in a reduction in platelet aggregation
Effect last for 7 days
Why is prostacylin production less inhibited by COX action
endothelial cells are nucleated so can synthesise more COX whereas the non-nuclear platelet can’t
How does clopidogrel work as an antiplatelet drug?
It irreversibly blocks the ADP receptor (P2Y12) on the platelet cell membrane
The effect of clopidogrel also lasts 7 days until new platelets have been produced
What is von Willebrand factor (VWF) and what is it synthesised by
A glycoprotein synthesised by endothelial cells and megakaryocytes and circulates in plasma as multimers (proteins with >1 monomer) of different sizes
VWF mediates the adhesion of platelets to sites of injury and promotes platelet-platelet aggregation
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Is a specific carrier for factor vIII
Why and when is fibrin formation needed
The primary platelet plug is enough for small vessel injury but would fall apart in larger vessels, so fibrin formation is needed to stabilise the platelet plug
Coagulation
Thrombin is generated which cleaves fibrinogen to generate a fibrin clot that stabilises the platelet plug
Where are clotting factors made
Most are made in the liver
Factor VIII and VWF are made by endothelial cells
VWF is also made in megakaryocytes
Which vitamins do some factors rely on
Vitamin K for carboxylation of their glutamic acid residues which is essential for their function
II (prothrombin), VII, IX and X are the factors
- What is each step of coagulation characterised by?
The conversion of an inactive zymogen (proenzyme) into an active clotting factor by splitting 1/more peptide bonds and exposure of the enzyme active site
What are 2 factors in coagulation
V and VIII
Where do clotting factors work
On exposed phospholipid surfaces of platelets which help localize and accelerate these reactions
Calcium ions role in secondary homeostasis
Important in binding of clotting factors to phospholipid surfaces of platelets
What happens in the initiation phase
Tissue factor (TF) is exposed on the surface of endothelial cells, leukocytes and most extravascular cells in an area of tissue damage- TF is normally located at sites not usually exposed to blood normally, so when blood encounters TF at the injury site, this initiates coagulation
TF makes factor VII into VIIa which leads to activation of factors IX to IXa and X to Xa
This leads to activation of factor II (prothrombin) which generates a **small initial amount of factor IIa (thrombin)
Amplification phase
The small thrombin amount (IIa) mediates the activation of cofactors V into Va and VIII into VIIIa, factor XI into XIa and platelets
What happens in the propagation phase
Factor XI converts more factor IX to IXa,
IXa and factor VIIIa, amplifies the conversion of factor X to Xa leading to a rapid burst in IIa and therefore thrombin generation
This cleaves the circulating soluble fibrinogen to form the insoluble fibrin clot
Why are coagulation inhibitory mechanisms important
As they prevent blood from completely clotting which ensures coagulation is confined to site of injury
How is antithrombin involved in coagulation inhibition
It inactivates thrombin (IIa) and factor Xa
The action of antithrombin is potentiated by heparin- this occurs by the binding of antithrombin to endothelial cell-associated heparins
What does heparin do
It’s a mixture of glycosaminoglycan chains extracted from porcine mucosa and works indirectly by binding to and potentiating the action of antithrombin which inactivates thrombin and factor Xa
Heparin **is administered intravenously or by subcutaneous injection
Inactivation of thrombin requires larger chains of heparin to wrap around both antithrombin and thrombin
What does warfarin do
It’s a vitamin K antagonist and works by interfering with protein carboxylation- this reduces synthesis of functional factors II, VII, IX and X by the liver
Oral tablet and regular blood testing is needed
Takes several days as it reduces synthesis of coagulation factors rather than inhibiting existing factors
Derived from coumarin
Direct oral anticoagulants
Orally available drugs that directly inhibit either thrombin or factor Xa without the involvement of antithrombin
They usually don’t require monitoring
Fibrinolytic system
Plasmin, which circulates in its inactive zymogen form plasminogen
It’s activated by tissue plasminogen activator (t-PA) when both t-PA and plasminogen together bind to lysine residues on fibrin
The breakdown of fibrin produces fibrin-degradation products (FDPs)
Plasmin isn’t specific to fibrin and also breaks down fibrinogen and clotting factors Va and VIIIa for example
Plasmin is inhibited by antiplasmin which circulates in the blood, also inhibited by alpha-2-macroglobulin
What does thrombolytic therapy entail
Thrombolytic agents like recombinant t-PA are given to ischaemic stroke patients intravenously to make plasmin and lyse clots- this is time dependent however so t-PA needs to be given to eligible patients ASAP, preferably within the first hour of onset of symptoms
There’s a high risk of bleeding associated with use
Who can be given thrombolytic therapy
Patients with life threatening pulmonary emboli
What does antifibrinolytic drug tranexamic do
It’s a synthetic derivative of the amino acid lysine and binds to plasminogen which prevents plasminogen from binding to lysine residues on fibrin- this is competitive inhibition
This prevents activation of plasminogen to plasmin which would lead to fibrinolysis
Tranexamic acid is used to treat bleeding in trauma and surgical patients and patients with inherited bleeding disorders
- Has a similar structure to the amino acid lysine, (synthetic derivative of the amino acid lysine)
- Binds to lysine binding sites on plasminogen, which prevents plasminogen from binding to lysine residues on fibrin, thus acts as a competitive inhibitor
- Which prevents activation of plasminogen to plasmin which would lead to fibrinolysis
Intrinsic vs extrinsic model
Intrinsic-components in plasma
factors IX, X, XI, XII and cofactors VIII and V
Extrinsic-TF and factors VII, X and cofactor V
Prothrombin time tests for
Integrity if the extrinsic pathways
Blood collected to which sodium citrate is added which chelates calcium to prevent blood clotting
Sample spun and recombinant thromboplastin added to citrates plasma with calcium to start clotting reaction
When is PT prolonged
If there’s a reduction in activity of factors VII, X, V, prothrombin or fibrinogen
We see bleeding in factor VII deficiency
Internalized normalized ratio
A correction for the different thromboplastin sources used by different labs when PT is used to monitor Vit K antagonist anticoagulant therapy like warfarin- this is so everyone can obtain the same INR result for a given sample irrespective of the source of thromboplastin.
Activated partial thromboplastin tests for
Measures the integrity if the intrinsic pathways
Performed by the contact activation of factor XII by a surface like glass/using a contact activator like silica or kaolin- this activator is added along with phospholipid to citrated plasma and then calcium- the time taken to clot is measured
When is APTT prolonged
When there is a reduction in a single or multiple clotting factors
In which 3 deficiencies do we see bleeding with an isolated prolonged APTT
Factor VIII deficiency (haemophilia A)
Factor IX deficiency (haemophilia B)
Factor XI deficiency
In which deficiency is there non bleeding patients with an isolated prolonged APTT
factor XII
What can lead it bleeding
Reduction in platelet number
Reduction in function of platelets
Reduction in coagulation factors due to congenital diseas or drugs,liver disease,DIC
Increased fibrinolysis may be due to administration of thrombolytic therapy
What so thrombosis
Formation of a blood clot within an intact vessel that occurs due to obstruction of blood flow
What is Virchow’s triad of contributory factors to thrombosis?
1) Blood- dominant in venous thrombosis
2) Vessel wall- dominant in arterial thrombosis
3) Blood flow- complex, contributes to both arterial and venous thrombosis
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Which 3 changes in blood increase the risk of venous thrombosis
Reduced levels of anticoagulant proteins
Reduced fibrinolytic activity eg inhibition of plasminogen activation through PAI2 by the placenta
Increased levels of clotting factors or platelet
Which factor increases during pregnancy
VIII
Direct vs indirect inhibition
Direct-antithrombin,an inhibitor of thrombin
Indirect-inhibition of thrombin generation by protein c and s anticoagulant pathways
Primary haemostasis
1) endothelial injury where secretion of nitric oxide and prostaglandin stop and secretion of endothelin starts
2) damage to endothelial cells exposes collagen which release vin willebrand factor
3)platelet changes shape releases more von willerbrand factor,calcium,ADP,thromboxane A2.
ADP binds to p2y12 and thromboxane binds to thromboxane A2 receptor in a positive feedback loop
How does vitamin k deficiency affect PT and APTT
APTT is increased as vitamin k is needed to make factor 9 10 and 2
pT is increased as there is deufcuney of factor 7 and ten
Severe haemophilia
Severe’ haemophilia refers to undetectable plasma levels of factor IX (haemophilia B) or factor VIII (haemophilia A) . Henry receives infusions of recombinant factor IX concentrate . These are known as prophylaxis, and reduce the risk of spontaneous bleeding, which he would be prone to given the severity of his disease
Beta thalassemia
Autosomal recessive
Affects beta chains of haemoglobin
Causes microcytic anaemia as haemoglobin not produced efficiently
Blood transfusion given
