Haemostasis Flashcards
What does blood do?
flows within the vascular system, transporting oxygen, nutrients and hormonal information around the body and removing metabolic waste
Why is a balance in haemostasis important?
- coagulation
- prevent generalised thrombosis
- start the process of fibrinolysis
What is coagulation?
stimulation of blood clotting processes following injury, in which blood changes from its liquid state
What is thrombosis?
excessive or generalised blood clotting
What is fibrinolysis?
the breakdown of the clot as part of the process of healing
What does haemostasis result from?
- Contraction of blood vessels (vasoconstriction)
- Formation of an unstable platelet plug at the site of the vessel wall damage (primary haemostasis)
- Formation of a stable fibrin clot (secondary haemostasis/coagulation)
What is vasoconstriction?
Contraction of blood vessels
What is primary haemostasis?
Formation of an unstable platelet plug at the site of the vessel wall damage
What is secondary haemostasis/coagulation?
Formation of a stable fibrin clot
What is the sequence of haemostasis?
- vasoconstriction
- primary haemostasis
- secondary haemostasis
- fibrinolysis
What are platelets?
discoid, non-nucleated, granule-containing cells that are derived from myeloid stem cells
Where are platelets formed and what from?
bone marrow by the fragmentation of megakaryocyte cytoplasm
What is the circulating lifespan of platelets?
around 10 days
What is important in the platelets interaction?
The plasma membrane contains glycoproteins (GPs)
What happens after injury to the vessel wall?
platelets stick to the damaged endothelium
How do platelets stick to the damaged endothelium?
either directly to collagen via the platelet GPIa receptor
or indirectly via von Willebrand factor (VWF), which binds to the platelet GPIb receptor
What happens after the adhesion of platelets?
become activated and change their shape from a disc to a more rounded form with spicules to encourage platelet-platelet interaction
What happenes after platelets are activated?
They release of the contents of their storage granules
What are the 2 types of granules in platelets?
a-granules
dense granules
How are the contents of platelet granules released?
The platelet membrane is invaginated to form a surface-connected cannalicular system through which they are released
What are the components of the granules in platelets?
ADP, fibrinogen and von Willebrand factor
What do platelets produce when they are stimulated?
prostaglandin thromboxane A2 from arachidonic acid that is derived from the cell membrane
What is the role of thromboxane A2?
Has a role in platelet aggregation
it is a vasoconstrictor and is especially important during tissue injury and inflammation
What is thromboxane A2 synthesised from?
Arachidonic acid –(ASA + Cyclo-oxygenase)–> Cyclic endoperoxides
in platelets:
Cyclic endoperoxides–(thromboxane synthetase)–> thromboxane A2 —-> plt aggreg.
What happens when granular ADP is released and thromboxane A2 is generated?
positive feedback effect
^^
further platelet recruitement and activation
How do platelets get activated?
by binding respectively to the P2Y12 and thromboxane A2 receptor
What else does platelet activation cause?
a conformational change in the GPIIb/IIIa receptor (known as ‘inside-out’ or ‘flip-flopping’)
What does the conformational change in the GPIIb/IIIa receptor provide?
binding sites for fibrinogen
What does Fibrinogen binding to GPIIb/IIIa cause?
‘outside-in’ signalling which further activates the platelets
What is the key role of fibrinogen?
linking platelets together to form the platelet plug
How are the effects of fibrinogen counterbalanced?
by the active flow of blood and the release of prostacyclin (PGI2) from endothelial cells
What is prostacyclin?
a powerful vasodilator and suppresses platelet activation
When platelet aggregation is suppressed what is prevented?
inappropriate platelet aggregation
What is aggregation?
the formation of a number of things into a cluster
What is adhesion of platelets?
They bind to the Von Willebrand factor by Glplb
or to collagen by Glpla
What does the adhesion of platelets cause?
Release of ADP + thromboxane
What does the release of ADP + thromboxane cause?
Platelet aggregation
What is the sequence of platelet aggregation?
adhesion
release
aggregation
What are antiplatelet drugs used for?
prevention and treatment of cardiovascular and cerebrovascular disease
Which are the most commonly used antiplatelet drugs?
Aspirin and clopidogrel
How does aspirin work?
Inhibits the production of thromboxane A2
How does aspirin inhibit the production of thromboxane A2?
irreversibly blocking the action of cyclo-oxygenase (COX), resulting in a reduction in platelet aggregation
What is inhibited by cyclo-oxygenase?
prostacyclin production but endothelial cells can synthesise more COX whereas the non-nuclear platelet cannot
How long does the effect of a single dose of aspirin last?
around 7 days
Why does the effect of aspirin wear away?
most of the platelets present at the time of aspirin ingestion have been replaced by new platelets
How does clopidogrel work?
It irreversibly blocks the ADP receptor (P2Y12) on the platelet cell membrane
How long does the effect of clopidogrel last?
around 7 days until new platelets have been produced
What is the Von Willebrand factor (VWF)?
a glycoprotein that is synthesised by endothelial cells and megakaryocytes and circulates in plasma as multimers of different sizes
What does Von Willebrand factor (VWF) do?
It mediates the adhesion of platelets to sites of injury
promotes platelet-platelet aggregation.
What is the Von Willebrand factor (VWF) a carrier for?
factor VIII (FVIII)
What are the properties of the Von Willebrand factor (VWF)
Adhesive properties
What is coagulation also called?
secondary haemostasis
What is coagulation?
formation of the stable fibrin clot
What is the primary platelet plug sufficient for?
small vessel injury
What stabilises the platelet plug?
Fibrin formation
What do blood coagulation pathways centre on?
the generation of thrombin, which splits fibrinogen to generate a fibrin clot that stabilises the platelet plug at sites of vascular injury
Where are most of the clotting factors synthesised?
liver
Which clotting factors are synthesised in endothelial cells?
factor VIII and VWF
Where else is VWF made?
megakaryocytes and incorporated into platelet granules
Which factors are dependant on Vitamin K?
Factors II (prothrombin), VII, IX and X
Why do these factors need Vitamin K?
carboxylation of their glutamic acid residues, which is essential for the function of these clotting factors
How is each step characterised?
by the conversion of an inactive zymogen (proenzyme) into an active clotting factor
How is the proenzyme converted into an active clotting factor?
splitting of one or more peptide bones
exposure of the active enzyme site
Which are co-factors?
Factors V and VIII
How do clotting factors work?
they work on the exposed phospholipid surface of platelets, which helps to localise and accelerate these reactions
What is the role of Calcium ions?
binding of activated clotting factors to the phospholipid surfaces of platelets
What is the trigger to initiate coagulation?
tissue factor (TF) exposed on the surface of endothelial cells and leukocytes and on most extravascular cells in an area of tissue damage
Where is the tissue factor mainly located?
sites that are not usually exposed to the blood under normal physiological conditions
When does blood encounter TF?
sites of vascular injury
What does TF bind to?
It binds to factor VIIa
What does the binding of TF to factor VIIa lead to?
activation of factors IX to IXa and X to Xa–> leads to:
the activation of prothrombin
what does the activation of prothrombin generate?
a small initial amount of thrombin (factor IIa)
What is the initiation phase?
blood only encounters TF at sites of vascular injury. The binding of TF to factor VIIa leads to the activation of factors IX to IXa and X to Xa. This leads to the activation of prothrombin (factor II) to generate a small initial amount of thrombin (factor IIa).
What is the amplification phase?
small amount of thrombin mediates the activation of the co-factors V and VIII, the zymogen factor XI and platelets
What does thrombin activate?
co-factors V and VIII, the zymogen factor XI and platelets
What is the propagation phase?
Factor XI converts more factor IX to IXa, which in concert with factor VIIIa, amplifies the conversion of factor X to Xa, and there is consequently a rapid burst in thrombin generation
What does thrombin split?
It splits the circulating fibrinogen (soluble) to form the insoluble fibrin clot
What is a natural anticoagulant pathway?
A number of inhibitory mechanisms prevent blood from clotting completely whenever clotting is initiated by vessel injury
What are the most important inhibitory mechanisms?
proteinC, protein S and antithrombin
What does the binding of thrombin to thrombomodulin?
(on the endothelial cell surface
activation of protein C to activated protein C (APC)
Where does the binding of thrombin to thrombomodulin happen?
on the endothelial cell surface
What does activated protein C (APC) do?
inactivates factors Va and VIIIa in the presence of a co-factor protein S
what does the inhibitor antithrombin do? (whilst circulating in the blood)
it inactivates thrombin and factor Xa
What does heparin do?
significantly increases the action of antithrombin by the binding of antithrombin to endothelial cell-associated heparins
What are the main anticoagulant drugs?
heparin, warfarin and the direct oral anticoagulants (DOACs)
Why are anticoagulant drugs used?
prevention and treatment of thrombosis
What does heparin consist of?
Heparin is a mixture of glycosaminylglycan chains extracted from porcine mucosa
How does heparin work?
indirectly by potentiating the action of antithrombin leading to the inactivation of factors Xa and IIa (thrombin)
How is thrombin inactivated?
requires longer chains of heparin chains, which are able to wrap around both the antithrombin and thrombin
How is heparin administered ?
intravenously or by subcutaneous injection
What is Warfarin?
derived from coumarin, is a vitamin K antagonist that works by interfering with protein carboxylation
How does warfarin work?
reduces synthesis of functional factors II, VII, IX and X by the liver
How is warfarin administered?
oral tablet and its anticoagulant effect needs to be monitored by regular blood testing
How long does warfarin take to start working?
several days bc it reduces synthesis of coagulation factors rather than inhibiting existing factor molecules
What are Direct oral anticoagulants (DOACs)?
directly inhibit either thrombin or factor Xa (i.e. without the involvement of antithrombin)
How are Direct oral anticoagulants (DOACs) administered?
orally
do not require monitoring
What happens after haemostasis has been achieved?
the body has a mechanism to break down (lyse) clots
what is the principle finrinolytic enzyme?
plasmin, which circulates in its inactive zymogen form plasminogen.
How is plasmin activated?
tissue plasminogen activator (t-PA)
When is plasminogen activated by t-PA?
when they are both brought together by binding to lysine residues on fibrin
What does the breakdown of fibrin lead to?
the generation of fibrin-degradation produces (FDPs)
what is the sequence of fibrinolysis?
plasminogen–(tissue plasminogen activator, tPa–> plasmin–> fibrin clot–> fibrin degradation products, FDP
Is plasmin specific?
No.
can also break down other protein components of plasma, including fibrinogen and the clotting factors Va and VIIIa
How is plasmin inhibited?
by antiplasmin which circulates in the blood
How does thrombolytic therapy works?
work by generating plasmin to lyse clots
How is thrombolytic therapy administered?
intravenously to selected patients presenting with ischaemic stroke
is thrombolytic therapy time-dependant?
Yes so t-PA needs to be given to eligible patients as quickly as possible, preferably within one hour of the onset of symptoms
Are there any risks associated with thrombolytic therapy?
high risk of bleeding
Who else is given thrombolytic therapy?
patients with life threatening pulmonary emboli and was previously used in patients with myocardial infarction, although this has largely been replaced with angioplasty and the insertion of stents to open the diseased coronary vessels
What is tranexamic acid?
synthetic derivative of the amino acid lysine that works by binding to plasminogen
ANTIFIBRINOLYTIC DRUGS
What happens when tranexamic acid binds to plasminogen?
prevents plasminogen from binding to the lysine residues of fibrin.
–> COMPETITIVE INHIBITION
What does competitive inhibition prevent?
prevents the activation of plasminogen to plasmin, which would otherwise result in fibrinolysis
What is tranexamic acid used for?
used widely to treat bleeding in trauma and surgical patients as well as in patients with inherited bleeding disorders
What replaced the classical ‘intrinsic’ and ‘extrinsic’ coagulation cascade model?
cellular-based model
initiation, amplification and propagation
What is the ‘intrinsic’ coagulation cascade model?
a system in which all components are in the plasma (factors XII, XI, IX, X and co-factors VIII and V)
What is the ‘extrinsic’ coagulation cascade model?
comprises TF and factors VII, X, and co-factor V
Is the intrinsic-extrinsic model still helpfull?
understanding of the blood tests used to assess coagulation
Intrinsic pathway
HMWK CONTACT PK XII --> XI --> VIII IX --> V X --> II -->in the conversion of fibrinogen to fibrin
Extrinsic pathway
VIIa
TF –>
V X –>
II –> in the conversion of fibrinogen to fibrin
What is prothrombin time?
Measures the wholeness and undividedness of the ‘extrinsic’ pathway
How is Prothrombin time measured?
Blood is collected into a bottle containing sodium citrate (usually blue-topped as in the picture), which chelates calcium thus preventing the blood from clotting in the bottle
The sample is spun to produce platelet-poor plasma
A source of TF and phospholipid is added to the citrated plasma sample, together with calcium to start the reaction; the length of time taken for the mixture to clot is recorded.
The PT may be prolonged if there is a reduction in the activity of factors VII, X, V, II (prothrombin) or fibrinogen i.e. (‘prothrombin’ is a misnomer)
What is the International normalised ratio (INR)?
When the PT is used to monitor vitamin K antagonist anticoagulant therapy such as warfarin the results are expressed as the international normalised ratio (INR)
What does INR involve?
a correction for the different thromboplastin reagents used by different laboratories and means that all laboratories would be expected to obtain the same INR result for a given sample irrespective of the source of thromboplastin
What is activated partial thromboplastin time? (APTT)
Measures the integrity of the ‘intrinsic’ pathway
How is APTT performed?
by the contact activation of factor XII by a surface such as glass, or using a contact activator such as silica or kaolin
What is the procedure of APTT?
Contact activator, together with phospholipid, is added to the citrated plasma sample followed by calcium; the time taken for this mixture to clot is measured
Where is prolongation of APTT seen?
in a variety of situations where there is a reduction in a single or multiple clotting factors
Where is an isolated prolonged APTT seen?
in patients with haemophilia A (factor VIII deficiency), haemophilia B (factor IX deficiency) and factor XI deficiency
may also be caused by factor XII deficiency which does not result in bleeding
What does the loss of haemostasis balance result in?
Bleeding
Bleeding can be caused by..
Reduction in platelet number or function (primary haemostasis –platelet plug)
Reduction in coagulation factor(s) (secondary haemostasis – fibrin clot)
Increased fibrinolysis
What is thrombosis?
the term used to describe the formation of a blood clot within an intact blood vessel
What does thrombosis usually result in?
obstruction of the blood flow with serious and possibly fatal consequences
What are the 3 contributory factors to pathological clotting (or thrombosis)?
(Vichrow’s triad)
Blood: dominant in venous thrombosis
Vessel wall: dominant in arterial thrombosis
Blood flow: complex, contributes to both arterial and venous thrombosis
What is the 1st change in blood that increase the risk of venous thrombosis?
- a Reduced levels of anticoagulant proteins (usually genetic)
- b Reduced fibrinolytic activity (pregnancy where there is inhibition of plasminogen activation through the production of a specific inhibitor by the placenta (PAI-2))
What is the 2nd change in blood that increase the risk of venous thrombosis?
Increased levels of clotting factors or platelets
