Haemostasis Flashcards
Coagulation factors
Help clotting
Regulatory factors
Stop clotting
Haemostasis at rest
Factors+ cofactors= seperated
Triggers for clotting outside blood (collagen+ tissue factor)
vWF
Activation?
von Willebrand factor= Thrombulin
Biggest soluble protein in blood
Several binding sites
Normally= rolled up (binding sites aren’t exposed) but when bound to collagen, opens up through blood flow
Platelets are fragments of?
Megakaryocytes (giant cells in bone marrow) Granular structure (bigger than vWF)
Platelet structure
Sticky adhesive receptors:
Integrin αIIbβ3- binding site for it on the vWF
GP1ba complex- binding site for it on the vWF
GP1a-IIa - bind to collagen
GPVI complex- bind to collagen
Stimulatory receptors:
P2Y1- ADP binds to this (P= purine)
P2Y12- ADP binds to this
PAR 1 & 4- ADP binds to this
Thromboxane receptor- thromboxane binds to this
PGI2 receptor- Prostacyclin binds to this
Granules:
Dense granule
Alpha granule
Lysosyme
Mitochondria
Open canallicular system
Formation of a platelet plug- Primary haemostasis
When is this sufficient?
- Primary haemostasis- rolled up vWF gets stretched out and binds to collagen.
- Exposes binding sites to platelets so platelets bind to vWF
- Platelets get activated and release the granules which release more vWF which captures more platelets etc
- Fibrinogen helps link platelet/ vWF complex together
Forms a platelet plug
Sufficient in small vessels (large vessels require secondary haemostasis)
Activated platelet
Change shape
Expose negatively charged phospholipid
Present new or activated proteins on their surface (i.e. GpIIb/IIIa)
Platelet binding
1) Platelet binds to vWF by GP1bα
2) When platelet slows down, can bind to collagen via GP1a-IIa+ GPVI too, and also to fibrinogen which binds it to other platelets via GPIIb-IIIa= causes Ca2+ INFLUX
3) Ca2+ influx causes activated GPIIb-IIIa+ degranulation which releases Fgn vWF (Fibrinogen vWF) and ADP
4) Ca2+ influx takes Phospholipid from the surface of the platelet (PLA2) and makes thromboxane (TXA2)
5) The release of ADP and thromboxane causes more platelets to come because receptors for ADP and thromboxane= activated (positive feedback event)
Secondary haemostasis Site of synthesis of molecules required in this process? Process? What is required? What isn't required?
Liver (most of it)
Endothelial cells that line vessels themselves
Megakaryocytes
Tissue factor required
Collagen not required
Secondary haemostasis PROCESS Trigger? Damping mechanism of trigger? Draw? (slide 17, lecture 17) What actually causes coagulation?
1) Trigger= tissue factor x VIIa complex formation
2) This turns zymogens 9 and 10 into active form (proteinase 9a and 10a)
3) Damping mechanism of trigger: TFPI= tissue factor pathway Inhibitor, which turns off the mechanism almost immidietaly by binding to 10a x tissue factor x VIIa to create an inactive complex
This means that this way of activating coagulation is turned off, but the 10a can make a little bit of thrombin
4) If you’ve made enough thrombin it will cause coagulation= conversion of fibrinogen (bulk protein in abundance in blood that is soluble) to fibrin (insoluble, forms a mesh)
5) If enough thrombin is made it will activate two other cofactors 5 and 8 into 5a and 8a, which help form 2 complexes:
a) 1st complex: Ca2+, 9a= enzyme, 8a= cofactor, PL= phospholipid the platelets are providing (link between primary and secondary haemostasis)
b) 2nd complex: 10a= enzyme, 5a= cofactor, PL, Ca2+
6) Extra feedback to make some 9 into 9a if needed
Result= amplification of thrombin production= platelet plug supported by fibrin mesh net that is cross linked by factor 13
Importance of thrombin production
Stronger, denser clot= more resistant to fibrinolysis
Factor XIII is activated by thrombin- cross links fibrin+ inhibits fibrinolysis
Thrombin activates TAFI = inhibitor of fibrinolysis
Anticoagulant function of endothelium
Proteins on surface of endothelium stop clotting in those areas (because the damage isn’t done there)
PGI2 and NO released from cells stop platelet activation
ADPase on membrane degrades ADP which stops the contribution of ADP in bringing more platelets etc to the site
Ensures clotting doesn’t occur everywhere
Inhibitory mechanisms of coagulation
Direct inhibition
Indirecti inhibition
Direct Inhibition: Deals with thrombin mainly
a) Antithrombin, inactivates XIa, IXa, Xa
Heparin makes the molecule more reactive and helps it and thrombin come together better
b) TFPI – in the initiation phase- damps down the 10a and 7a effect
Indirect inhibition: Deals with cofactors mainly
Inhibition of thrombin generation by the protein C anticoagulant pathway:
Thrombomodulin on endothelial membrane modulates thrombin’s activity.
1) Binds free thrombin and redirects thrombin activity from fibrinogen to protein C.
2) Activated Protein C + cofactor protein S down-regulates thrombin generation by degrading Va and VIIIa= stops thrombin production
Termination and localisation of coagulation
The thrombin that tries to get away causes APC production
Binds to Antithrombin which is either free floating or on heparan which is on endothelial membrane
Fibronolysis
1) tPA needs to meet Pgn to turn Pgn into plasmin, they meet on fibrin.
2) Plasmin is powerful enzyme that targets fibrin which forms FDPs (fibrin degradation products)
3) Antiplasmin stops the use of this outside the localised area
Risk factors for thrombosis?
Genetics?
Deficiency of anticoagulant proteins:
Antithrombin deficiency
Protein C deficiency
Protein S deficiency
Increased coagulant proteins/activity: Factor VIII Factor II & others Factor V Leiden (increased activity due to activated protein C resistance)- hereditary defect, resistant to Protein C Thrombocytosis (increased platelets)
Heterogenous genetic defect= viable
Homogenous genetic defect= death at birth
SBA
Platelets
1.Bind to damaged endothelium through tissue factor.
2.Are inhibited by thromboxane.
3.Trigger coagulation at the site of injury.
4.Are activated and aggregate in response to ADP.
5.Have a life span of 100 days.
4
Coagulation:
- FVIIIa is a protease, generated by thrombin.
- Factor XII interacts with tissue factor to initiate coagulation.
- Activated coagulation complexes assemble on the platelet surface.
- Fibrinogen must be converted to fibrin by thrombin in order to support platelet aggregation.
- FIXa is a cofactor that accelerates FX activation.
3
Normal haemostasis
Balance between: Anticoagulant proteins: AT, PC, PS Fibrinolytic factors: tPA, Pgn Antiplatelet factors: PGI2 Endothelium: TM, EPCR, TFPI, Heparan
AND Coagulation factors I-XI Fibrinolysis inhibitors: AP, PAI-1, FXIII, TAFI Platelets Inflammatory mediators: IL-6, TNF
Characteristics of abnormal bleeding
‘Spontaneous’
Out of proportion to the trauma/injury
Unduly prolonged
Restarts after appearing to stop
Summary process of when you have an injury
1) Vessel constriction
Vascular smooth muscle cells contract locally
Limits blood flow to injured vessel
2) Primary Haemostasis: Formation of an unstable platelet plug
Platelet adhesion (vWF etc) + aggregation (ADP+ thromboxane)
Limits blood loss + provides surface for coagulation
3) Secondary Haemostasis: Stabilisation of the plug with fibrin
Blood coagulation
Stops blood loss
4) Fibrinolysis- Vessel repair and dissolution of clot
Cell migration/proliferation & fibrinolysis
Restores vessel integrity
Defects of primary haemostasis (primary plug formation)
Examples of each?
Defective collagen- vessel wall (e.g.Steroid therapy, age, scurvy)
Defective Von Willebrand factor (e.g. Von Willebrand disease (genetic deficiency))
Platelets can’t stick- can’t bind to collagen if they go at speed
Can’t form the plug and even if the coagulation system works there is nothing for it to hold onto
Defective platelets (e.g. Aspirin & other drugs, Thromobocytopenia)
Petichiae
Typical of thrombocytopenia
Pinprick type bleeding spots
Shows that vessels constantly break and reform
