1: Haemostasis Flashcards
2 factors necessary for maintaining circulation of blood in fluid state to blood vessels
Fibrinolytic anticoagulant proteins
Coagulation factors, platelets
Structure of platelets (3)
Discoid
Non-nucleated
Granule-containing
Where and how are platelets formed
Bone marrow
Fragmentation of megakaryocytic cytoplasm from myeloid stem cells
Haemostasis is
Halting of blood following trauma to blood vessels
2 functions of haemostasis
Prevention of blood loss from intact vessels
Arrest of bleeding from injured vessels
Balance in haemostasis is important for (3)
Coagulation
Prevents thrombosis
Fibrinolysis - break down of clot as part of healing process
Mechanism of haemostasis
1) Response to injury ( vessel constriction)
2) [1°] unstable platelet plug formation (platelet adhesion and aggregation)
3) [2°]stabilisation of platelet plug with fibrin (blood coagulation)
4) [fibrinolysis] dissolution of clot and vessel repair (cell proliferation + fibrinolysis)
Primary haemostasis is
Formation of unstable platelet plug
- platelet adhesion
-platelet activation
-platelet aggregation
Platelets are
Granule cont. cells
Made from myeloid stem cells
Plasma membrane contains glycoproteins
Lifespan of platelets
10 days
Formation of unstable platelet plug
- Platelet adhesion
- Platelet aggregation
Process of Platelet adhesion
Collagen exposed on endothelium of damaged vessel wall
Platelets stick to collagen directly via GP1a
Stick to VWF (Von willebrand factor) which binds to GP1b on collagen
Adhesion activates platelets
Process of platelet activation
platelets activated after adhesion
releases alpha granules and dense granules (storage)
stored granules also release ADP, VWF, fibrinogen
platelets produce thromboxane A2 from arachidonic acid (plasma membrane)
conformational change in GP2b/3a receptor
process of platelet aggregation
Thromboxane A2 and ADP have positive feedback on platelet recruitment, activation and aggregation
ADP binds to P2Y12 receptor and thromboxane A2 binds to thromboxane A2 receptor
GP2b/3a allows binding of fibrinogen and calcium
fibrinogen links platelets to form unstable platelet plug
Arachidonic acid produces ______ which inhibits platelet adhesion and aggregation, how?
Prostacyclin (PGI)
- released from endothelial cells
- vasodilator
- supresses platelet activation to avoid inappropriate aggregation
Why is dosage of antiplatelet drugs important
platelets have a lifespan of 10 days
dose lasts around 7 days until all platelets are replaced
2 antiplatelet drugs
aspirin
clopidogrel
How does aspirin act as an antiplatelet drug
Irreversibly blocks COX (enzyme)
inhibits thromboxane A2 production
so platelet activation inhibited
[PGI production also inhibited by blocking COX, but endothelial cells can produce more COX, but platelets can’t)
How does Clopidogrel act as an antiplatelet drug
Irreversibly blocks ADP receptor (P2Y12) on platelet cell membrane
Secondary haemostasis is
formation of stable fibrin clot
Reason for stabilisation of primary platelet plug
sufficient for small vessel injury
in larger vessels it falls apart
fibrin formation stabilises it
Process of primary platelet plug stabilisation
Blood coagulation generates thrombin
cleaves fibrinogen to generate fibrin clot, stabilising platelet plug at site of injury
Synthesis of clotting factors
most made in liver (12 total)
factor VIII and VWF made by endothelial cells
VWF also made in megakaryocytes and put into platelet granules
How do clotting factors produce fibrin
work in a cascade to produce fibrin
What is Vitamin K needed for in secondary haemostasis
carboxylation of factors II, VII, IX and X (2,7,9,10)
How do clotting factors work
bind to exposed phospholipid surfaces of platelets -helps localise and accelerate reaction
Ca2+ important for binding of clotting factors to surfaces
convert inactive zymogen (proenzyme) into active clotting factor
factors V and VIII are co-factors
3 Phases of Coagulation (cellular based model)
initiation phase
amplification phase
propagation phase
In the initiation phase of coagulation
Tissue factor (TF) exposed on endothelial cell surface post injury
TF binds to 7a — activates 9— 9a and 10— 10a — activation of prothrombin (factor2)— small amount of thrombin (factor 2a)
In the amplification phase of coagulation
small amount of thrombin activates cofactors 5 and 8, zymogen factor 9 and platelets
In the propagation phase of coagulation
activated factor 9 (9 to 9a) with 8a form 10 to 10a - rapid burst in thrombin
thrombin cleaves soluble fibrinogen to form insoluble fibrin clot
3 Natural anticoagulants
confine coagulation to site of injury only
- protein C, protein S and antithrombin
How do protein C, S and antithrombin act as natural anticoagulants
-thrombin binds to thrombomodulin on endothelial cell surface
leads to activation of protein C to activated protein C (APC)
APC inactivates factors Va and VIIIa in presence of co-factor protein S
-thrombin and factor Xa inactivated by circulating inhibitor antithrombin
3 Anticoagulant drugs
Heparin
Warfarin
DOACs (Direct oral anticoagulants)
How does Heparin work as an anticoagulant drug
Works indirectly by potentiating action of antithrombin leading to inactivation of factors Xa and IIa (thrombin)
administered intravenously or by subcutaneous injection
How does Warfarin work as an anticoagulant drug
Vit K antagonist : interfere with protein carboxylation
reduces synthesis of functional factors, 2,7,9,10 by liver
Given as an oral tablet, effects monitored by regular blood testing
several days to work
How do DOACs work as an anticoagulant drug
Orally available drugs that directly inhibit either thrombin or factor Xa
Do not require monitoring
Fibrinolysis is
Vessel repair and dissolution of clot
Main fibrinolytic enzyme
plasmin
How does plasmin work
Plasminogen (inactive zymogen form) circulates in blood
Tissue plasminogen activator (t-PA) and plasminogen bind to lysine residues on fibrin
break down fibrin into fibrin degradation products (FDPs)
2 things inhibiting plasmin
Antiplasmin (circulating in blood)
alpha 2 macroglobulin
Thrombolytic therapy is
artificial fibrinolysis
Example of thrombolytic therapy
Recombinant t-PA generate plasmin to lyse clots, break down fibrin to FDPs
- administered intravenously to patients presenting with ischaemic stroke
- needs to be given ASAP, preferably within 1h to onset of symptoms
- can be administered to patients with pulmonary emboli
Function of antifibrinolytic drugs
- stops body breaking fibrin clot
- keep clots = bleed less
Effects of Tranexamic acid as an antifibrinolytic drug
Antifibrinolytic drug derived from lysine
binds to plasminogen and prevents it from binding to lysine on fibrin
Competitive inhibitor
- treats: bleeding in trauma, surgical patients, patients with bleeding disorders
Test of coagulation - common pathway
Factors V and X form factor II
Which converts Fibrinogen to Fibrin
Test of coagulation - Extrinsic pathway
PT (prothrombin time)
VII a converts TF into factor X
then follows common pathway
Test of coagulation - Intrinsic pathway
APTT
PK - XII
converted into XI
converted into VIII and IX
both converted to factor X and follow common pathway
INR is
standardising method of Prothrombin time for comparison
Prothrombin time PT measures
integrity of extrinsic pathway
How to measure PT
- Blood collected in bottle containing sodium citrate - chelates calcium- preventing clotting in bottle
- sample spun to produce platelet-poor plasma
-source of TF, calcium and phospholipid added to start reaction - length of time taken for mixture to clot is recorded
Activated partial thromboplastin time (APTT) measures
integrity of intrinsic pathway
Prolonged APPT and normal Pt seen in what 4 conditions
Haemophilia A (FVIII deficiency)
Haemophilia B (FIX deficiency)
Factor XI deficiency
Factor XII deficiency - no bleeding
How to measure APTT
Contact activation of factor XII by glass or using silica or kaolin
contact activator, together with phospholipid, added to citrated plasma sample followed by calcium
time taken for mixture to clot is measured
Prolonged PT signifies
reduction in FVII, X, V, prothrombin or fibrinogen activity
3 Things that increase bleeding
- Reduction in platelet number (thrombocytopenia) or function (primary haemostasis - platelet plug)
- Reduction in coagulation factors (secondary haemostasis - fibrin clot)
- Increased fibrinolysis
Virchows triad
3 most important factors in blood clot formation:
Blood - dominant in venous thrombosis
Vessel wall - dominant in arterial thrombosis
Blood flow - complex and contributes to both
Venous thrombosis is
Blood clots forming in veins
2 main things that occur in venous thrombosis
Decrease in fibrinolytic factors and anticoagulant proteins
Increase in coagulation factors and platelets
What factors cause a decrease in fibrinolysis and anticoagulant proteins in venous thrombosis
pregnancy - inhibition of plasminogen, activation through placental production of inhibitor PAI-2
genetics - inherited antithrombin deficiency
what factor increases coagulation factors and platelets leading to venous thrombosis
levels of FVIII increase in pregnancy
FV leiden - mutation in FV gene makes it more resistant to inactivation by protein C
Myeloproliferative disorders cause incr. in platelet output by bone marrow
