Haemostasis Flashcards
what is the haemostatic response to endothelial injury [4]
- vessel constriction
- formation of unstable platelet plug
- stabilisation of plug with fibrin
- dissolution of clot and vessel repair
what occurs with platelets in unstable plug formation?
adhesion and aggregation
what is the function the endothelium?
- Maintain barrier between blood and tissue factor.
- Synthesis of PGI2, thrombomodulin, vWF, plasminogen activators
what is the platelet transformation process from stem cell to release from the bone marrow?
origin: bone marrow
- haemopoietic stem cell precursors
- myeloid stem cell
- megakaryocytes precursor
- undergoes nuclear replication without cytoplasmic division
- matures and granulate
- migrates to marrow sinusoids
- megakaryocytes produce ~4000 platelets by fragmenting into circulation through the endothelial wall of the bone marrow
what is the lifespan of a platelet?
~10 days
how much platelets is stored/sequestered in the spleen?
1/3
platelet features
what are the glycopeptides on the surface?
Dense granules
alpha-granules
thrombin receptors
surface glycoproteins: Glp1a, Glp1b, Glp2b/3a).
what glycoprotein of a platelet does vWF bind to?
Glp1b
what does Glp1a bind to?
collagen
what needs to be released to cause enhanced aggregation?
what is the effect?
ADP (released from platelets granules) and thromboxane from other platelets
causes Glp2b/3a receptor expression on the platelets
what do endothelial cells and platelets produce mainly
endothelial- PGI2 (prostacyclin)
platelets- TXA2
what enzyme makes the final PGI2 product?
prostacyclin synthetase
what enzymes makes the final TXA2 product?
thromboxane synthetase
what effect that PGI2 have on platelets?
potent inhibitor of platelet function
remember prostacyclin is anti-thrombotic
what are the effects of endoperoxides (PGG2 and PGH2) and thromboxanes on platelets?
potent inducer of aggregation
what enzyme does aspirin affect in prostaglandin metabolism?
cyclo oxygenase (COX1)
[converts arachidonic acid to endoperoxides]
prevents TXA2 production so it can’t activate platelet aggregation
examples of COX1 inhibitors
aspirin
examples of ADP receptor antagonists
clopidogrel (P2Y12)
prasugrel
example of Glp2b,3a antagonists
Abciximab
Tirofiban,
Eptifibatide
drug classes used in antithrombotic therapy in the ARTERIAL system?
COX-1 Inhibitors/antagonists
ADP receptor antagonists
GlpIIb/IIIa antagonists
i.e. antiplatelets
blood is too fast for coagulation due to stasis to happen
examples of lab tests
platelet count (monitor thrombocytopenia)
bleeding time and platelet aggression
what does ITP present with
Auto-immune Thrombocytopenia (ITP) presents with – purpura, multiple bruises, ecchymoses.
example of when platelet count would be used in monitoring treatment?
In treatment of leukaemia’s, you can knock out platelets.
what does bleeding time reveal?
check platelet-vessel-wall interactions, when the platelet count is normal, e.g. renal disease.
what does platelet aggregation reveal?
measures functional defect of platelets, e.g. vWF disease, inherited defects.
what are the sites of clotting factor, fibrinolytic factor and inhibitor synthesis?
- liver (coag.proteins, antithrombin)
- endothelial cells (VWF)
- megakaryocytes (VWF, Factor V)
order of factors in intrinsic pathway until thrombin formation
12,11,9, 10
where does TF get involved?
in the extrinsic pathway with F7
what is tested for to show the intrinsic pathway
F12
what factors drive the positive feedback loop from the intrinsic pathway into the common pathway?
8a (for 9a) and 5a (for 10a)
phospholipids found on platelet surface membrane
what does F9a activate?
via which factor?
F10 via F8aPI
what does F10a activate?
via which factor?
F2 (prothrombin to thrombin) via F5aPI
what is the effect of thrombin apart from fibrin formation?
- drive more platelet activation
- more expression of F8a and F5a PIs
thrombin generation is accelerated by x10000 by F8a and F5a PI
which vitamin mediates the production of factor 2, 7, 9, 10?
where is this done?
what extra group do these factors have?
Vitamin K
in the liver
extra carboxyl group
what is the effect of the extra carboxyl group on the factors?
allow adherence to the platelet membranes phospholipids
what is the extra carboxyl unit on the factors?
gamma-carboxyglutamic acid
what enzyme does warfarin inhibit?
what is the effect of this?
vitamin K epoxide reductase
reduced secretion of clotting factors and their binding e.g. f10
interactions to membrane PL is reduced as they can’t bind with Ca2+
what is the indication for warfarin?
long-term anticoagulation following venous thrombosis
treatment of atrial fibrillation
stroke prophylaxis
DVT treatment
what is the effect of Heparin?
binds to antithrombin and speeds it up 3000x
what is the indication for heparin?
immediate/ acute anticoagulation in peripheral oedema and DVT
MI
unstable angina
pulmonary embolism
what factors does antithrombin affect?
F11a, F10a, F9a, F2a(thrombin)
how does antithrombin have its anticoagulation effect
binds to factors irreversibly by altering their tertiary structure e.g. bind to active site
[9a, 10a, 2a]
structure of heparin
negatively charged string-like structure
what are the forms of heparin that can be used?
when are they used
1) low MW
2) high MW
both can be used to inhibit F10a
only high MW can be used to inhibit F2a (thrombin) as It anchors the antithrombin to it better
which lab test detects abnormality in the intrinsic (and common) pathways?
how does it do this?
Activated Partial Thromboplastin Time (APTT)
- initiates coag. via F12
what lab test detects abnormality in the extrinsic (and common) pathways?
how does it do this?
Prothrombin Time (PT)
- initiates coag. via TF
what lab test detects abnormality in fibrinogens conversion to fibrin?
Thrombin Clotting Time (TCT/TT)
- add thrombin (activated thrombin facilitates the conversion)
what tests are used to detect bleeding disorders?
APTT
PT
what test is used to monitor heparin therapy in thrombosis?
APTT as intrinsic pathway needs to be monitored
what test is used to monitor warfarin therapy in thromboses?
PT as extrinsic pathway and common pathway need to be monitored
what is required for the production of plasmin from plasminogen?
Tissue Plasminogen Activator (TPA) and a fibrin clot
without the clot, the plasminogen (zymogen) does not interact to create plasmin
what is the role of plasmin?
fibrin degradation (fibrinolysis)
activated from plasminogen by tPA
what components of fibrinolysis are used therapeutically for thrombolysis in MI?
TPA (alteplase)
bacterial activator streptokinase
what factors are inhibited by antithrombin in the coag. inhibition mechanism?
F9a
F10a
F2a (thrombin)
what factors are inhibited by Protein S and C in the coag. inhibition mechanism?
F8a and F5a
how is protein S and C activated?
when thrombin binds to platelets to create thrombomodulin which then activates protein S and C
how does the prothrombin time change in haemophilia?
prolonged
i.e. clotting time has increased
which factors does the anticoagulants warfarin inhibit the activation of?
2,7,9,10
all them key players
why is prothrombin time used to measure the effect of warfarin?
warfarin impacts factor 7
factor 7 is part of the extrinsic pathway involving tissue factor
the tissue factor/extrinsic pathway is best measured using prothrombin time
how are the effects of heparin measured?
using activated partial thromboplastin time (APTT)
- intrinsic pathway and common pathway measurement
- heparin enhances the actions antithrombin (an enzyme inhibitor glycoprotein produced in the liver)
- antithrombin can deactivate thrombin, f10 etc
what are 3 examples of antiplatelets?
1) aspirin–> COX-1 therefore TXA2 reduced
2) clopidogrel–> ADP receptor
3) tirofiban–> GlpIIb/IIIa inhibitor
in acute management of MI
nb warfarin and heparin are antiCOAGULANTS (efficacy in secondary haemostasis)
what is the role of TXA2?
expression of GlpIIb/IIIa
released by platelet activation
what is the function of the haemostatic plug?
prevent blood loss from intact vessels
arrest bleeding from injured vessels
what is the first response to injury?
vessel constriction
temporarily restrict blood loss
what molecules activate platelets leading to formation of unstable plug?
ADP
thrombin
TXA2 (made from membrane phospholipids)
how is the unstable plug stabilised?
by coagulation (initiated by Tissue Factor)
this produces the fibrin clot
what vessels disorders arise due to impaired interaction of platelets with the injured vessels?
- scurvy
- senile purpura
- allergic vasculitis (acquired)
- haemorrhagic telangiectasia
- Ehlers-Danlos syndrome (inherited).
