Haemostasis Flashcards
State the steps involved in haemostatic plug formation from the time of injury.
Vessel constriction
Formation of an unstable platelet plug (platelet adhesion + platelet aggregation)
Stabilisation of plug with fibrin (blood coagulation)
Dissolution of clot and vessel repair (fibrinolysis)
What component found underneath the endothelium is involved in triggering the coagulation cascade?
Procoagulant subendothelial structures e.g. collagen Tissue factor is also expressed on the surface of the cell that underlie blood vessels but it is NOT normally expressed within the circulation itself
State some important factors produced by endothelial cells.
Prostacyclin
Thrombomodulin
Von Willebrand Factor
Plasminogen Activator
What process during maturation of the megakaryocytes is important for the formation of platelets?
Granulation
How many platelets are produced by one megakaryocyte?
4000
What do the dense granules in platelets contain that is important for platelet function?
ADP
What do alpha granules in the platelets contain?
vWF
Factor V
State the two ways in which platelets can bind to collagen. Name the receptors involved.
It can bind via vWF to collagen (via the GlpIb receptor)
It can bind directly to the collagen (via the GlpIa receptor)
What happens following the passive adhesion of platelets andengagement of receptors?
The receptors signal inside the cell to release ADP from the storage granules and to synthesise thromboxane
These bind to receptors on the surface of the platelets and activate them
Once activated, GlpIIb/IIIa receptors become available, which can bind to fibrinogen and allows the platelets to aggregate
Which receptors on the platelets become available following activation of the platelets and what do they bind to?
GlpIIb/IIIa
These bind to fibrinogen
What else can activate platelets?
Thrombin
Which enzyme converts phospholipids to arachidonic acid?
Phospholipase
What does COX convert arachidonic acid to?
Endoperoxides
Describe what happens to endoperoxides in platelets and in the endothelial cells.
Platelets – thromboxane synthetase converts endoperoxides to thromboxane (potent inducer of platelet aggregation)
Endothelial Cells – prostacyclin synthetase converts endoperoxides to prostacyclin (important regulator of haemostasis)
What effect does aspirin have on this entire pathway? What effect does aspirin have on this entire pathway?
Aspirin is a COX1 inhibitor
State some important drug targets in platelet aggregation.
COX
GlpIIb/IIIa
ADP Receptor
State two ADP receptor antagonists.
Clopidogrel
Prasugrel
State three GlpIIb/IIIa antagonists.
Abciximab
Tirofiban
Eptifibatide
What is the most important test for monitoring platelets and their function?
Platelet count
What is a common cause of spontaneous bleeding?
Autoimmune thrombocytopenia (autoimmune antibodies clear platelets from the circulation) This results in purpura, multiple bruises and ecchymoses
What is the normal range for platelet count?
150-400 x 109/L
Why do you get thrombocytopenia in leukaemia?
Leukaemic cells populate the bone marrow so it crowds out the megakaryocytes so the platelets aren’t produced in sufficient numbers
What is the bleeding time test used to observe?
This checks the platelet-vessel wall interaction
This isn’t used any more
Describe the platelet aggregation test.
The platelets are stimulated with ADP/thromboxane/collagen to study their function
This is used to diagnose platelet disease e.g. von Willebrand disease
Where is von Willebrand factor produced?
Endothelial cells and a little bit by megakaryocytes
What factors do megakaryocytes produce?
Factor V
Von Willebrand Factor
Tissue factor activates the clotting cascade by converting 9 to 9a and by converting 10 to 10a. What difference does this make?
9 to 9a – slower but produces more thrombin
10 to 10a – faster
State two accelerating factors. What are they activated by?
Factor VIII
Factor V
They are activated by trace amounts of thrombin
Which factors are activated on the surface of the platelet? Describe how this works.
10 to 10a
2 to 2a (prothrombin to thrombin)
For 9a to activate 10 it needs to come in close proximity with 10. They both bind to the surface of the platelet mediated by calcium ions, andfactor VIIIa bring the two close together so that 9a can proteolytically cleave 10 to 10a
Factor Va does the same with 10a and 2 (prothrombin)
Which factors are affected by warfarin?
2, 7, 9, 10
What is common to all of these factors and what is the significance of this common feature?
They have a cluster of glutamic acid
The glutamic acid is recognised by an enzyme in the liver and undergoes post-translational modification in the presence of vitamin K to Gamma-carboxyglutamic acid
Once this extra carboxyl group is added, calcium can facilitate the binding of gamma carboxyglutamic acid to the activated platelet membrane phospholipid
How does warfarin actually inhibit the post-translational modification of these factors?
Warfarin inhibits vitamin K epoxide reductase thus inhibiting the gamma carboxylation of factors 2, 7, 9 and 10
Which factors are inhibited by anti-thrombin?
2, 9, 10, 11
What effect does heparin have on anti-thrombin?
Heparin potentiates the action of anti-thrombin
In what situation is heparin used?
Heparin is used for immediate anticoagulation in venous thrombosis and pulmonary embolism
Describe how anti-thrombin inhibits the clotting factors.
Anti-thrombin has a reactive loop that irreversibly inhibits the active site on the clotting factors
So anti-thrombin acts as a scavenger in stopping inappropriate action of clotting factors
How does heparin potentiate the action of anti-thrombin and why is the chain length important?
Heparin is a linear negatively charged polysaccharide
Once bound to anti-thrombin it changes the position of the reactive loop and makes the inhibition occur faster
When inhibiting factor 10a, a relatively SHORT chain of heparin is sufficient (low molecular weight heparin)
When inhibiting thrombin, you require a LARGE chain (standard/unfractionated heparin)
NOTE: standard/unfractionated heparin inhibits either Factor 10a or thrombin
State three laboratory tests for blood coagulation.
Activated Partial Thromboplastin Time (APTT)
Prothrombin Time (PT)
Thrombin Clotting Time (TCT)
What do each of these laboratory tests represent?
APTT – detects abnormalities in the INTRINSIC and COMMON pathways (coagulation is triggered by activation of factor 12)
PT – detects abnormalities in the EXTRINSIC and COMMON pathways (tissue factor is added to trigger the extrinsic pathway)
TCT – shows abnormality in the fibrinogen fibrin conversion (not important any more)
What are the main uses of these laboratory tests?
APTT and PT are used together for screening causes of bleeding disorders
APTT is used to monitor heparin therapy for thrombosis
PT is used to monitor warfarin treatment
What two proteins assemble on the surface of a clot to allow fibrinolysis to take place? Where are these proteins made?
Plasminogen
Tissue Plasminogen Activator (tPA)
Plasminogen is a plasma protein
tPA is produced by endothelial cells
What is produced from the break down of the fibrin clot and how does this level change in DIC?
Fibrin degradation products (FDP)
This is elevated in DIC
What factors are used in therapeutic thrombolysis of myocardial infarction?
tPA and bacterial activator streptokinase
Describe the function of the Protein C anticoagulant pathway.
Thrombin also has a role in anticoagulation
It binds to thrombomodulin on the surface of endothelial cells and by binding to thrombomodulin it activates protein C, which, along with co-factor protein S, INACTIVATES FACTOR Va and FACTOR VIIIa
