Haemostasis Flashcards
How is normal haemostasis of the blood maintained and using which factors?
A state of equilibrium is established between fibrinolytic factors (anti-coagulant proteins) and coagulation factors (platelets)
Why is this balance important? (3 things this system does)
- Coagulation - stimulation of blood clotting processes following injury
- Thrombosis - Limit response to the site of injury (to prevent excessive blood clotting)
- Fibrinolysis - Start the process of the breakdown of the clot
Haemostasis following trauma to blood vessels:
- Vasoconstriction
- Primary haemostasis - formation of an unstable platelet plug using platelet adhesion and platelet aggregation
- Secondary haemostasis/coagulation - formation of a stable fibrin clot
- Fibrinolysis - dissolution of clot and vessel repair
What is haemostasis?
Prevention of blood loss
Where are platelets derived from and what is their lifespan?
From myeloid stem cells, which differentiate to megakaryocytes, which fragment to form platelets.
Have a lifespan of 8-10 days
What are the 4 steps of primary haemostasis?
- Platelet adhesion
- Platelet activation/ release action
- Thromboxane A2 synthesis
- Platelet aggregation
The method of platelet adhesion following injury?
Platelets stick to the damaged endothelium in two ways:
- The GPIa receptor on the platelets directly attaches to the damaged vessel
- The GPIb receptor on the platelets binds to von Willebrand factor (VWF) which is attached to the damaged vessel
What happens during platelet activation / release reaction?
The adhesion of the platelets activates them - they release the contents of their storage granules including ADP, Ca2+, fibrinogen and VWF
The GPIa and GPIb receptors also change shape so they can bind to fibrinogen
How is thromboxane A2 synthesised?
From arachidonic acid, using the cyclooxygenase enzyme
Why is the release of thromboxane A2 important?
Release of ADP and generation of thromboxane A2 have positive feedback effects resulting in further platelet recruitment activation and aggregation
How are platelets aggregated?
Using fibrinogen, which binds to platelets and links them together to form the platelet plug
How is this process counteracted / returned to normal?
Prostacyclin released from the endothelial cells causes vasodilation, and suppresses platelet activation and consequently prevents (inappropriate) aggregation
What are 2 common anti-platelet drugs?
Aspirin and Clopidogrel
How does Aspirin work?
Aspirin inhibits the cyclooxygenase enzyme to block thromboxane A2 production, which reduces platelet aggregation
How long do the effects of aspirin last?
7 days - until the platelets at the time of aspirin ingestion have been replaced by new platelets
How does Clopidogrel work?
Irreversibly blocks the ADP receptor (P2Y12) on the platelet cell membrane, again reducing platelet aggregation
How long do the effects of Clopidogrel last?
Same as Aspirin - 7 days
What is secondary haemostasis (coagulation) and why is it important?
It is the formation of a fibrin clot that stabilises the initial primary platelet plug, which would otherwise fall apart (especially in large vessels)
What are the steps for the formation of the fibrin clot?
- Activate coagulation factor
- Set off coagulation cascade
- Production of fibrin from fibrinogen
- Formation of fibrin mesh
What are the methods by which coagulation factors are activated?
- Intrinsic pathway (all components in the plasma) - subendothelial collagen (protein) lies under endothelial cells unexposed to the blood, so when the cell is damaged, this protein is released into the blood, activating coagulation factors that trigger the coagulation cascade
- Extrinsic pathway - Tissue Factor (TF) is secreted by endothelial cells when they are damaged, activating coagulation factors that trigger the coagulation cascade
What is the coagulation cascade?
Following the activation of one coagulation factor, consequent coagulation factors are activated, eventually leading to the production of fibrin. It goes through the initiation, amplification and propagation phases, leading to a rapid burst of thrombin
Which vitamin is important for the coagulation factors?
Vitamin K
What is the difference between fibrin and fibrinogen and how is fibrin producted?
Fibrinogen is fibrin with a protein attached to it, so after the coagulation cascade, the enzyme thrombin cleaves the fibrinogen to form fibrin
Which 2 things are essential for the formation of the fibrin mesh?
It requires Ca2+ (released during primary haemostasis), and the phospholipid surface of the platelet
Why is Ca2+ important?
Has an important role in the binding of activated clotting factors to the phospholipid surfaces of platelets
Why is the phospholipid surface important?
Links the formation of the fibrin mesh with the initial plug - forming the stable clot
Why are there coagulation inhibitory mechanisms?
Prevent blood from clotting completely and blocking the blood vessel
Ensures coagulation is confined to the site of injury and prevent the spontaneous activation of coagulation
What are the 2 coagulation inhibitory systems?
Anti-coagulation and thrombolysis
What is anti-coagulation?
Prevention of blood clots forming
Which parts of the coagulation process can anti-coagulation work on?
It can either prevent primary haemostasis or secondary haemostasis
How can primary haemostasis be prevented?
Healthy endothelial cells release molecules that prevent coagulation, such as prostacyclin and nitric oxide, which prevent platelets from binding to the endothelial cells, and cause vasodilation of the blood vessel
How can secondary haemostasis be prevented?
- A heparin like molecule interacts with the inhibitor, antithrombin, and together they inhibit the enzyme thrombin (and factor X). This reduces the production of fibrin from fibrinogen, so a fibrin mesh cannot be formed
- Thrombin binds to thrombodulin on the endothelial cell, activating protein C, which in the presence of protein S inactivates other coagulation factors
What is artificial anti-coagulation?
Anti-coagulant drugs e.g. heparin, warfarin, and DOACs (direct oral anticoagulants)
What are anti-coagulant drugs used to treat?
Thrombosis
How is heparin taken and how does it work?
Intravenously, it upregulates the action of antithrombin
How is warfarin taken and how does it work?
Orally, as a tablet. It is a vitamin K antagonist, hence prevents the synthesis of coagulation factors (rather than inhibiting existing factor molecules), it takes several days to take effect
How are DOACs taken and how do they work?
Direct oral anticoagulants (DOACs) are orally available drugs that directly inhibit either thrombin or factor Xa (i.e. without the involvement of antithrombin)
What does the fibrinolytic system do?
Carries out fibrinolysis, the break down of the clot
What is the principle enzyme in the fibrinolytic system and how does it circulate around the body?
Plasmin
Circulates in its inactive form, plasminogen
What activates plasminogen?
Tissue plasminogen activator (t-PA) activates plasminogen only when brought together by binding on lysine residues on fibrin
What does the breakdown of fibrin produce?
Leads to the generation of fibrin-degradation produces (FDPs)
What else can fibrin break down and how is this controlled?
Other proteins in the plasma e.g. fibrinogen and some coagulation factors.
Antiplasmin circulates the blood and inhibits plasmin
What is thrombolytic therapy?
Administering thrombolytic agents to break up the clot. Used to treat life threatening pulmonary emboli and/or ischaemic strokes
How do the thrombolytic agents work?
Agents such as t-PA work by generating plasmin to lyse clots and are administered intravenously to selected patients
When do these thrombolytic agents need to be given to be effective?
As quickly as possible, preferably within one hour of the onset of symptoms
Risks of thrombolytic therapy?
Bleeding
What is a common antifibrinolytic drug?
Tranexamic acid
How does Tranexamic acid work?
Competitive inhibitor
Synthetic derivative of lysine, and binds to plasminogen
This means the lysine residues on the fibrin are unable to bind to the plasminogen to activate it to plasmin
Reduced plasmin generation results in suppression of fibrinolysis
Where are antifibrinolytic drugs used?
To treat bleeding in trauma and surgical patients as well as in patients with inherited bleeding disorders
What are the 2 methods to test coagulation time?
- Prothrombin time (PT)
2. Activated partial thromboplastin time (APTT)
What is the process of measuring Prothrombin time (PT)?
Blood collected in a bottle containing sodium citrate, which forms chelates with Ca2+ to prevent blood clotting in the bottle
The sample is spun to produce platelet-poor plasma
TF, phospholipids (or nowadays, recombinant thromboplastin is often used as the source of both TF and phospholipid) and Ca2+ are added to the citrated plasma sample, the length of time taken for the mixture to clot is recorded
Why might the Prothrombin time (PT) be prolonged?
Reduction in the activity in the coagulation factors (specifically Factor II, Factor V, Factor XII or Factor X) or fibrinogen
When PT is used to monitor anticoagulant agents (E.g. warfarin), the units are?
International normalised ratio (INR)
What is the process to measure activated partial thromboplastin time (APTT)
Contact activator (e.g. glass or silica) to activate factor XII, phospholipid and calcium is added to the citrated plasma sample. The time taken for this mixture to clot is measured
Why might the activated partial thromboplastin time (APTT) be prolonged?
Reduction in a single or multiple clotting factors -Factor VIII or IX
Which factor deficiency does not cause bleeding?
Factor XII
What are 3 possible causes of bleeding?
- Reduction in platelet number or function (primary haemostasis –platelet plug)
- Reduction in coagulation factor(s) (secondary haemostasis – fibrin clot)
- Increased fibrinolysis
Why might there be a reduction in platelet number?
- Failure of platelet production: drugs, viruses, bone marrow infiltration, megaloblastic anaemia resulting from B12 or folate deficiency, hereditary thrombocytopenia
- Shortened platelet survival – immune thrombocytopenia, disseminated intravascular coagulation (DIC)
- Increased splenic pooling
Why might there be a reduction in platelet function?
- Antiplatelet drugs e.g. aspirin
2. Inherited causes
What are the 2 broad reasons to why blood abnormalities may arise?
- Congenital
2. Acquired
Why might there be a reduction in coagulation factors?
Congenital causes:
1. von Willebrand disease (VWD) - autosomal - reductions in the level or function of von Willebrand factor (VWF)
2. Haemophilia A - factor VIII deficiency, X linked – males affected, females carriers
3. Haemophilia B - factor IX deficiency, X linked – males affected, females carriers
Acquired:
1. Liver disease
2. Aniticoagulant drugs
3. Disseminated intravascular coagulation (DIC) - TF activated = uncontrolled activation of coagulation = activation of the fibrinolytic system = large amounts of thrombin released = huge consumption of platelets = fibrinolysis activation = high levels of fibrin degradation products (FDPs)
What may trigger DIC?
Many reasons including bacterial sepsis, advanced cancer and a variety of obstetric emergencies
How can DIC be treated?
Replacement of missing clotting factors and platelets may help control the bleeding symptoms (but root cause also needs to be dealt with for long term management)
Why might there be increased fibrinolysis (causing bleeding)?
- DIC
2. Thrombolytic therapy - anticoagulant drugs
What are the 3 contributory factors to thrombosis in ‘Virchow’s triad’?
- Blood: dominant in venous thrombosis
- Vessel wall: dominant in arterial thrombosis
- Blood flow: complex, contributes to both arterial and venous thrombosis
What changes in the blood can increase the chances of a venous thrombosis?
- Reduced levels of anticoagulant proteins (usually genetic basis e.g. thrombophilia)
- Reduced fibrinolytic activity (e.g during pregnancy plasminogen is inhibited)
- Increased levels of clotting factors or platelets
