Haemostasis Flashcards
In tact endothelial function is needed for normal circulation.
Which naturally occurring substances prevent thrombus formation on an intact endothelium? (5)
- Nitric oxide
- Prostacyclin
- Heparans
- Tissue factor pathway inhibitor (TFPI)
- Thrombomodulin
What happens when you cut yourself?
- You bleed at the site of the injury
- The bleeding stops and you form a clot => Platelets, vWF, Coagulation Factors
- The clot remains confined to the site of injury => natural Anticoagulants
- And then about 1 week later the clot has vanished => Fibrinolytic System
When there is damage to the endothelial lining a clot is required.
Q. What are the 2 key signals that let resting platelets and coagulation factors know that they need to become activated to start the process of clot formation?
Abnormal surface – damaged vessel wall leads to rough collagen surface and the platelets recognise this abnormal surface and stick to it to plug the hole
Physiological activator - tissue factor - when you damage a blood vessel you release a small amount of the normal physiological activator of coagulation into the circulation
How are endothelial cells connected to subendothelial collagen?
By Von Willebrand factor (VWF)
What important structures do platelets have on their surface that helps them to do their job in clot formation? (2)
Cell surface receptors:
- ADP receptor
- Epinephrine receptor
- Thrombin receptor
When erythrocytes and endothelial cells are damaged they release ADP and Epinephrine into the circulation and form thrombin. These then bind to receptors on the platelet surface and allow it to behave normally and make a clot.
Platelet glycoproteins
These are binding sites for important ligands that are involved in coagulation such as:
- GP 2b/3a = fibrinogen
- GP 1b/5/9 = VWF
- GP 1a/2a and GP 6 = collagen
Platelets have an open canalicular system. What is meant by this?
- Platelets have canals that allow the content of the platelet to be released into the circulation.
- Within platelets there are dense and alpha granules and lysosomes that are released into the circulation as required to perform different functions.
- When platelets are activated the actin and myosin within contracts and squashes the cell, pushing the granules out through the canals onto its surface which makes it sticky.
What do dense granules produce? (3)
- ADP/ATP
- Calcium
- Serotonin
What do alpha granules produce? (2)
- Thrombin
- VWF
What are the 3 A’s in haemostasis?
- Adhere
- to collagen through GP 1a and GP 6
- to fibrinogen through GP 2b/3a
- to vWF through GP 1b
- Activation
- p2y12 pathway - activated by ADP that is released by damaged cells/tissues
- COX pathway - produces thromboxane which results in platelet aggregation
- Aggregation
- Thromboxane causes the platelets to aggregate
Which drug interferes/blocks the Cyclooxygenase (COX) pathway to prevent production of thromboxane?
Aspirin - prevents thromboxane production which prevents platelet aggregation (prevents the stickyness of platelets basically)
What is the p2y12 pathway?
- p2y12 is a chemoreceptor on the surface of platelets in which ADP binds to, triggering reactions within the cell.
- The P2Y12 receptor is involved in platelet aggregation
Which drugs inhibit/block the p2y12 pathway? (3)
- Clopidogrel
- Prasugrel
- Ticagrelor
These are commonly used in patients who have had a STEMI or PCI
When people speak about dual anti-platelet therapy what are they referring to?
The use of aspirin with one of the p2y12 inhibitors therefore blocking both the COX and p2y12 pathways
What does the enzyme scramblase do and why is it important?
Scramblase allows the phopholipid layer to be expressed on the external surface to act as a site for coagulation reactions can take place on
Why is vWF so important in clot formation?
vWF is basically a big sticky molecule that you want around when making a clot because:
- It binds to factor 8 which you always want to make a clot
- It binds to the surface of platelets through GP 1b
- It binds to collagen through a couple of receptor sites
Describe the process of platelet plug formation. This happens within 30 seconds to a minute of injury.
- Platelets adhere to the damaged vessel - bind to collagen and to von W factor
- The platelets become activated (p2y12) and start to aggregate together to form a platelet plug (COX pathway)
- Once the platelets become activated they slightly change their conformation which allows them to bind to fibrinogen via GP 2b/3a - this happens at the same time as the other steps
However, next step => need to get fibrin as platelets are not enough.
How is a fibrin clot formed after formation of a platelet plug?
- The clotting cascade is activated
- The fibrinogen bound to the platelets is cleaved to form fibrin
- This creates a fibrin clot which stops the bleeding
Normal clotting cascade
Normal clotting sequence will act like dominos – cascade starts and if everything is in place then fibrinogen will be cleaved to form fibrin net over the platelet plug and this forms the stable clot
Defective clotting cascade
- If there is an abnormality in the clotting sequence the clotting sequence starts but when it gets to a point where there is absent or reduced clotting factor then it all goes wrong.
- When that happens fibrinogen is not cleaved and the bleeding in the organ continues (haemophilia)
Haemophilia A is caused by a deficiency of which clotting factor?
Factor 8
Haemophilia B is caused by a dificiency of which clotting factor?
Factor 9
Describe the clotting cascade process.
- How does it start?
- What order does it go in?
- End product?
- Route/s?
- There are 2 paths, intrinsic and extrinsic which originate separately but converge at a specific point, leading to fibrin activation. Need both pathways as need factor 8 and 9 to stop bleeding
Extrinsic - immediate reaction which produces a small burst of thrombin but not enough to secure haemostasis
- Normal physiological activator = tissue factor is released when we damage tissue and this binds to factor VII and activates it starting the clotting cascade process
- Activated factor VII binds to factor X and activates it
- Factor X with a little bit of help from factor V activates prothrombin to form thrombin
- Thrombin cleaves fibrinogen sitting on surface of platelet to form fibrin
Intrinsic - fed by thrombin
- Factor 11 cleaved to 11a
- 11a cleaves 9 to 9a
- factors 9a and 8a cleave 10 to form factor 10a
- 10a acts on prothrombin bringing about a big burst of thrombin = stable clot + haemostasis
ALL FACTORS NEED TO BE THERE AND FUNCTIONING PROPERLY TO CREATE SECURE CLOT
Give 1 congenital, 1 aquired and 1 drug related reason as to why a patient may be missing coagulation factors?
- Congenital - haemophilia
- Acquired - liver disease
- Drug - anti-coagulants
Why does the clot stay confined to the area in which you cut yourself?
Due to natural anti-coagulant pathways
What are the 3 most important natural anti-coagulant systems?
- TFPI - tissue factor pathway inhibitor
- Activated protein C
- Anti-thrombin
Which clotting factors does TFPI bind to and inhibt?
Factor Xa and VIIa which means you don’t make much thrombin and you stop making blood clot
How does the activated protein C system work?
- Activated protein c binds to its cofactor protein s and these molecules switch off Factor VIII and factor V
- Again they switch coagulation off and reduce thrombin production thereby reducing clot formation
How does anti-thrombin work as a natural anti-coagulant?
- The most powerful of the natural anti-coagulants
- Binds to and inactivates lots of coagulation factors but particularly it binds and inactivates Factor X and thrombin
Look
Deficiency of Protein C+S and anti-thrombin increase your predispositon to the development of blood clots
- Those who are deficient don’t switch off coagulation activation in a successful way and form too many DVTs and PE
Look
After a secure clot has been formed and bleeding outside of the vessel has stopped we need to remodel the endothelium. We need to start to remove the clot to do so.
= Fibrinolysis
Once you produce plasmin to break down the clot you achieve tissue repair
After you form a clot, your endothelial cells respond by secreting 2 things. What are they?
They secrete the activators of plasminogen which are:
- t-PA
- u-PA
What do t-PA and u-PA do?
- They cleave plasminogen to form plasmin
- Plasmin attacks the clot and breaks it down
- Breaking down clot releases fibrin and fibrin degradation products
What is D-dimer?
The direct result of cleavage of cross linked fibrin by plasmin
What are the natural inhibitors of plasminogen?
- Plasminogen activator inhibitor-1 (PAI-1)
- Plasminogen activator inhibitor-2 (PAI-2)
What are the natural inibitors of plasmin?
Alpha 2 - antiplasmin
Alpha 2 - macroglobulin
Which anti-platelet drugs prevent the binding of fibrinogen to GP 2b/3a? (3)
- Abciximab
- Tirofiban
- Eptifibatide
These are commonly used in PCI situations where patient looks like they are continuing to thrombose their stent even though they’ve been on dual anti-platelet therapy and have had a dose of something like heparin to prevent this etc
Which Anticoagulate drugs are commonly used?
Heparins - unfractionated, LMWH - bind to anti-thrombin (natural anti-coagulant) and allows it to cleave thrombin and activated factor 10 more quickly
Warfarin - vitamin K antagonist - vit K is required for post-translational modification of clotting factors 2, 7, 9 and 10. So basically it reduces the function of these 4 clotting factors. It can be reversed by administration of vit K or prothrombin complex concentrate (concentrate including these missing clotting factors)
New drugs - Rivaroxaban, Edoxaban and Apixaban - sit on active site of activated factor 10 and prevent it from cleaving prothrombin to thrombin. So no fibrin produced and no clot formed
Dabigatran (oral) and Bivalirudin/Argatroban (parenteral) - directly inhibt thrombin
