Haemosatasis 1 Flashcards
What are the two potential outcomes of the haemostatic system
May stop you bleeding to death.
May kill you with thrombosis
What is the ultimate aim of clinical haemostasis
A balance between thrombosis and bleeding
What happens in an intact blood vessel
coagulation factors (denoted by roman numerals) help form plug, regulatory proteins stop premature firing to prevent thrombosis
Describe the platelets
fragments of cells made from megakaryocytes in bone marrow, containing granules and receptors
Where are the triggers for coagulation found though
Found outside the circulation- expressed on fibroblasts (collagen and tissue factors).
What is separated at rest in haemostasis
factors and cofactors separated
What is the von Wileband Factor
VWF is a giant adhesive plasma protein
with many binding sites for platelets (GP1b, GP2b3a), collagen, factor VIII; assembled to multimers (20-50 monomers); usually rolled up in blood so binding sites hidden
Biggest soluble protein in the blood.
Describe what the von Willeband Factor does in response to shear stress
The collagen binding sites are normally hidden- but the shear stress of blood stretches collagen and when it binds to collagen- it becomes long and thin-exposing binding sites
Must be bound to collagen first
Bumps and gaps represent monomer
Describe the key features of platelet structure
Adhesive receptors on surface can bind to vWF (GP1b complex/Integrin alpha-II-b-beta3) and collagen (P1a-Iia/GPVI complex)
Stimulatory receptors on surface can be activated by ADP (purine receptors e.g. P2Y1/12), thromboxane and PGI2
What can stimulate platelet activation
ADP, Thromboxane and PGI2.
Describe the histology of platelets
Much smaller than erythrocytes
Granular (ADP, fibrinogen, vWF).
What granules are present inside the platelet
Lysosome
Alpha granule
Dense granule
Describe the formation of the platelet plug
Endothelium damaged Blood meets collagen and tissue factor VWF in rolled up form VWF binds to collagen- stretched out Binding sites exposed for platelets Platelet Activation Degranulation- more VWF- more platelets trapped Fibrinogen links the platelets together Formation of primary platelet plug Coagulation system not used.
What is not required for primary haemostasis
Tissue factor
What happens in platelet activation
Conversion from a passive to an interactive cell
Activated platelets:
Change shape
Expose phospholipid- negatively charged phospholipid.
Present new or activated proteins on their surface (i.e. GpIIb/IIIa)
What can cause platelets to change shape
Shape changes during adhesion, activation and aggregation
What happens when platelets bind to the VWF
Platelets bind to VWF via GpIb
This slows down the platelet- allowing it to bind to collagen via Gp I- II alpha or the GPV1 complex. Platelets link together via fibrinogen via GPIIB-IIIa.
This binding activates the platelet and signals the activation of the platelet:
Dense granules release ADP
Alpha granules release Fibrinogen and VWF
Phospholipid exposed- TXA2 produced- which can act on the platelet
Brought about by Ca2+ influx.
What does primary haemostasis require
Requires
Collagen
Platelets
Von Willebrand Factor
Where is primary haemostasis sufficient
Sufficient for small vessels
In larger vessels requires stabilisation (fibrin)
How do we stabilise the plug formed in primary haemostasis
Coagulation:
Formation of a fibrin mesh
(secondary haemostasis)
Where are Clotting Factors, Fibrinolytic
Factors and Inhibitors synthesised
- The liver
- Endothelial cells (VIII and VWF)
- Megakaryocytes ( differentiate to platelets) (VWF ± FV)
Most synthesis is in the liver
What does ‘a’ mean after VIIa
Activated
Circulates in its active form- but useless until it meets its cofactor (tissue factor).
Describe secondary haemostasis
Endothelium damaged
Blood meets collagen and tissue factors (normally outside the vessel)
In 2o haemostasis, collagen not important, factor VIIa (activated factor VII - circulates in active form; useless unless tissue factor co-factor present) binds to co-factor
Tissue factor-VIIa complex forms, converting inactive FIX/FX zymogens to activated FIXa/Fxa
TFPI (Tissue factor pathway inhibitor) binds to FXa, FVIIa, and TF to sequester in inactive form to dampen and stop pathway as soon as begins
If large enough stimulus, FXa converts FII to thrombin
Thrombin forms FV and FVIII- which are cofactors for FXa and FIXa respectively- Ca2+, PL, FIXa, FVIIIa] which converts FX to FXa, and [Ca2+, PL, FXa, FVa] which converts FII to thrombin
What is thrombin
Thrombin cleaves fibrinogen (soluble and bulk) to fibrin (insoluble and forms mesh)
Describe the amplification effect of these complexes
cofactor and enzyme have massive amplification effect - 105 x normal activity, working in cascade to make thrombin from FII (prothrombin)
Describe the thrombin burst
Tissue stimulated with tissue factor- lag phase whilst things get activated
Massive spike in thrombin release- soon returns to normal as thrombin is mopped up
Less pronounced thrombin release in patients with Haemophilia A.
What is the importance of the thrombin burst
Essential for normal haemostasis A thrombin burst generates a stronger, denser clot which is more resistant to fibrinolysis Factor XIII is activated by thrombin cross links fibrin and inhibits fibrinolysis Deficiency of coagulation factor(s) causes a bleeding disorder due a to failure of thrombin burst Thrombin activates TAFI – an inhibitor of fibrinolysis
Describe secure haemostasis
primary haemostatic plug forms, and then secondary haemostatic net forms on top (made of fibrin linked by FXIII)
Summarise haemostasis
- Simply viewed, it is a cascade or amplification system
- There are zymogens (inactive) which are converted to proteinases,
cofactors which need to be activated and surfaces - The surface is made of activated platelets (Pl) which localise
and accelerate the reactions - The trigger to initiate coagulation in vivo is tissue factor
- Although FXII can be activated to FXIIa, this is mainly
an in vitro reaction, useful for some diagnostic tests
Is FXII physiologically necessary
No- but used for diagnostic tests- part of the contact activation system
No F12- you won’t bleed
No F8- you will bleed.
Why does blood not clot completely
Coagulation inhibitory mechanisms prevent this
Describe the anticoagulant function of the endothelium at rest
collagen/tissue factors kept separate from circulation; endothelial proteins are anti-thrombotic (heparin/heparans/PGI2, NO and ADPase)
Describe the regulation of completed coagulation
Direct Inhibition:
Antithrombin (sometimes known as antithrombin III), which
is an inhibitor of thrombin and other clotting proteinases
TFPI – in the initiation phase
Indirect inhibition:
Inhibition of thrombin generation by the protein C
anticoagulant pathway
This deals with the cofactors.
Describe the role of heparin
Heparin accelerates the action of
antithrombin
Makes it more reactive x1000.
Bridges thrombin and antithrombin activity
What is antithrombin
(i) Antithrombin is a direct inhibitor of thrombin
and other proteinases
antithrombin can then also clear activated enzymes (XIa, IXa, Xa)
Describe the role of protein C
ii) the protein C pathway down-regulates
thrombin generation
TM binds free thrombin
TM redirects thrombin function from cleaving fibrinogen and producing FV and FVIII to protein C.
Activated Protein C + cofactor protein S down-regulates
thrombin generation by degrading Va and VIIIa- forming Vai and VIIIai
EPCR presents protein C to TM
What will the thrombin trying to escape also meet
Binds to AT on heparans- or in free solution
Coagulation system cannot propagate further.
Response is limited to the site where it is needed.
What is fibrinolysis
process of degrading the clot.
Describe the process of fibrinolysis
Localised response
tPA (tissue plasminogen activator) converts Pgn (plasminogen) to form Plasmin when meet on Fibrin
Plasmin (enzyme) cleaves fibrin to fibrin degradation products (FDPs)
Antiplasmin stops plasmin moving to rest of system and degrading all fibrin (to localise)
What happens if the coagulation inhibitory mechanisms fail
Inappropriate clot formation = thrombosis DUE TO Antithrombin deficiency Protein C deficiency 3. Protein S deficiency
Describe normal haemostasis
Equilibrium
Anti-coagulant factors:
Anticoagulant proteins
AT, PC, PS
Fibrinolytic factors
tPA, Pgn
Antiplatelet factors
PGI2
Endothelium
TM, EPCR, TFPI, Heparan
Pro-coagulant factors:
Coagulation factors I-XI
Fibrinolysis inhibitors
AP, PAI-1, FXIII, TAFI
Platelets
Inflammatory mediators
IL-6, TNF
What is thrombosis
Inappropriate clot formation- in an intact vessel- pathological.
