haemostasis 1: haemostasis Flashcards
what is haemostasis?
the cellular and biochemical processes that enable the specific and regulated cessation of bleeding in resonse to vascular insult
what is the purpose of haemostasis?
to prevent blood loss from intact & injured vessels and enable tissue repair
how does a haemostatic plug form?
1) vessel constriction - vascular smooth muscle cells contract locally -> limits blood flow to injured vessel
2) formation of unstable platelet plug - platelets adhere + aggregate -> limits blood loss + provides surface for coagulation
3) stabilisation of plug with fibrin (associated with cascade) - blood coagulation -> stops blood loss
4) vessel repair and dissolution of clot - cells migrate + proliferate and fibrinolysis takes place -> restores vessel integrity
what is the structure of a blood vessel (in terms of coagulation properties)?
endothelial layer = anticoagulant, subendothelium = procoagulant
what is Von Willebrand factor?
glycoprotein that circulates in globular conformation with platelet binding sites ‘hidden’
how does the structure of VWF change in response to vessel injury?
globular VWF recognises collagen -> binds -> structural change to linear conformation -> platelet binding sites exposed -> platelets bind to Gp1b
what are some important surface proteins οf platelets?
GPVI (interact with collagen), αIIbβ3 integrin that interacts with fibrinogen), α2β1 (integrin that interacts with collagen) GP1b (essential for capture by VWF)
how does a platelet plug form?
platelets bind to GP1b on VWF
- > recruited to site of vessel damage
- > interact with collagen
- > collagen acts as stimulator of platelets
- > platelet activated
- > change shape + release granules + membrane changes conformation
- > platelets stick together (αIIbβ3)
when can platelets directly interact with collagen?
low shear stress (ie not in arteries & capillaries)
how does the shape of a platelet change upon activation?
flowing disc-shape -> ball-shaped on rolling -> hemisphere-shaped (firm but reversible adhesion) -> spreading platelet (like fried egg) - irreversible adhesion
what happens in Von Willebrand disease?
not enough / dysfunctional VWF -> initial platelent recruitment step inefficient
what do platelet disorders cause?
dysfunctional / not enough platelets -> bleeding phenotype
below what platelet count is spontaneous bleeding common?
<40x10^9 (below 10x10^9 is severe)
what is the major function of thrombin?
cleaves fibrinogen (soluble) -> fibrin (insoluble)
where are most circulating clotting factors synthesised?
liver (endothelial cells + megakaryocytes also produce clotting factors)
what are the main groups of clotting factors?
zymogens eg prothrombin, FVII, FX (activated to become serine proteases eg thrombin, FVIIa, FXa), cofactors eg TF, FVa, FVIIIa, inhibitors eg TFPI, protein C
what do serine proteases have in common?
homologous serine protease domains
what is tissue factor?
integral membrane protein that is the primary initiator of coagulation & is cell receptor for FVII - located at extravascular sites + expressed more in certain organs eg lungs, brain, heart
-> provides further haemostatic protection in these organs
what is factor VII?
serine protease zymogen produced by liver - has Gla domain, 2 EGF-like domains + serine protease domain (1% circulates in active FVIIa form)
what features do FVII, FIX, FX & PC share?
homologous modular structure (4 domains), Gla domain, EGF domain, all circulate in zymogen form, all activated by proteolysis
what does a Gla domain do?
allows factors to interact with negatively charged phospholipid layers
which factors contain Gla domains?
FVII, FX, prothrombin, FIX, protein C, protein S
what is warfarin and how does it achieve its function?
vitamin K antagonist diminishes response of clotting factors (in Gla domains:
glutamic acid residues post-translationally modified by vitamin K dependent carboxylases
-> additional carboxylic acid added to glutamic acid
-> gives residues affinity to Ca2+
-> domains fold up in such a way that allows binding of phospholipids)
what is the first main step of the clotting cascade?
FVIIa binds TF (via Gla domain) -> activates FIX or FX (by cleaving activation peptides)
what does FXa do in the clotting cascade?
FXa liberates thrombin (from ProT) - very inefficient as only small quantities of thrombin are generated
what happens when initial thrombin is generated in the clotting cascade?
thrombin feedbacks to FVIII & FV -> FVIIIa (cofactor for FIXa) & FVa (cofactor for FXa)
what effects do FVIIIa & FIXa then have in the clotting cascade?
lots more FXa produced
what cofactors in the clotting cascade then result in further thrombin production?
FXa + FVa -> 300,000x more thrombin released (compared with FXa alone)
what is the overall effect of initial thrombin release in the clotting cascade?
small amounts of thrombin positively feedbacks on coagulation cascade to rapidly produce much more thrombin
what anticoagulant pathways regulate the clotting cascade?
TFPI (tissue factor pathway inhibitor - initiation), APC & PS (FVa + FVIIIa), antithrombin (FXa + FIXa)
how does TFPI work?
2nd domain binds to FXa -> complex rebinds to TF-FVIIa complex -> 1st TFPI domain binds to FVIIa -> locks all 4 proteins in inactive form
what is the TFPI response mainly used for?
dampening response to very small injuries (only small amount of circulating TFPI)
what happens in the protein C pathway?
thrombin binds with high affinity to thrombomodulin on endothelial cell -> becomes anticoagulant -> complex activates protein C -> APC cleaves FVa + FVIIIa at different locations -> cofactors fall apart
what overall effect does the protein C pathway have?
“fences” clot ie where endothelium is present clot will not form as anticoagulant thrombin-TM complex present
what happens to the rest of the thrombin in the clot?
antithrombin (a serine protease inhibitor (SERPIN)) inactivates many activated coagulation serine proteases eg Fxa, thrombin, FIXa, FXIa - essentially mops up free serine proteases that escape site of vessel damage
how does heparin work?
acts as a cofactor for antithrombin: binds to AT -> enhances efficiency with which antithrombin can inhibit thrombin + FXa
what happens in fibrinolysis?
tPA (tissue plasminogen activator) binds to fibrin -> becomes activated -> plasminogen converted to plasmin -> degrades fibrin clot -> fibrin degradation products removed by liver
what drugs can be used to reduce haemostasis?
anticoagulants (heparin, warfarin, DOACs) & antiplatelet drugs (aspiring, P2Y12 blockers) - often anticoagulants given for protection against venous thrombi and antiplatelets for arterial thrombi
what tests can be used to diagnose abnormal haemostasis?
PT & APTT (blood test looking at coagulation of blood), platelet function tests (analysis of platelets), d-dimer (fibrin degradation product)
how can tPA be used therapeutically?
to treat MI, ischaemic stroke etc
