Haemostasis Flashcards
define Haemostasis
a dynamic process where blood coagulation is rapidly initiated at the site if injury to prevent blood loss and promote injury repair and healing
what are the 3 main haemostatic functions
maintentance of blood in a fluid state within the vascular system.
to arrest bleeding at the site of injury by formation of a haemostatic plug.
removal of the plug when healing is complete and a return to fluidity
what happens haemostatically in a normal physiology
a delicate balance between clotting and bleeding and therefore deficiency or exaggeration of any one may lead to either thrombosis or haemorrhage
what causes bleeding in the balance
when the balance is shifted to bleeding
what happens when the balance is shifted towards clotting
thrombosis
what are the three phases of haemostasis
primary - primary platelet plug formation
secondary - stable clot formation
tertiary- fibrinolysis (clot dissolution)
what are the components involved in haemostasis
blood vessels - endothelium platelets plasma proteins -coagulation factors -their inhibitors -fibrinolytic system
how intact endothelium naturally anti-thrombogenis
the endothelium isolates blood from clot-promoting substances which is beneath the endothelium
name some pro-coagulant molecules
von Willebrand Factor
Collagen
Tissue Factor
Plasminogen Activator Inhibitor 1 (PAI-1)
name some anti-coagulant molecules
nitric oxide (NO) prostacyclin (PGI2) tissue factor pathway inhibitor (TFPI) tissue plasminogen activator (t-PA) heparan sulphated proteoglycans (HSPG)
whats the overview of platelets
anuclear, small (2-3um), cellular fragments highly refractile and disk-shaped
platelet membrane - phospholipid bilayer, receptor sites
granules
dense
alpha
microfilaments - pseudopodia
microtubules - contraction
what are three plasma proteins
coagulation factors
natural inhibitors
fibrinolytic system
what are coagulation factors
inactive enzymes or zymogens: predominately serine proteases
activated by cleavage: exposes the active site
substrate of active factor: zymogen of another
numbered by roman numerals and a indicates active
in order of discovery not order of activation
many of the coagulation factors require co-factors to function effectively
what is the classic model of coagulation (1905)
prothrombin catalysed by thrombokinase +calcium ions leads to thrombin
thrombin causes fibrinogen to become fibrin
what is the cascade/ waterfall model of coagulation (1964
there are two pathways
the intrinsic (contact) pathway
extrinsic (tissue factor) pathway
the third pathway came later and thats the common pathway
what happens in the intrinsic pathway
factor XII, HMWK and prekallikrein activate FXI
FXIa activates FIX
FIXa combines with FVIII and phospholipid: activates FX
ensues to final common pathway
what happens in the extrinsic pathway
tissue factor and FVIIa form a complex (TF:FVIIa): activates FX
TF:FVIIa also activates FIX (intrinsic) helps increase FXa
ensues to final common pathway
what happens in the common pathway
prothrombin activator produced on surface of activated platelets: FXa \+FVa \+phospholipid \+calcium ions
the activator cleaves prothrombin (FII) into thrombin (FIIa)
thrombin (FIIa) induces fibrin (FIa) polymers from Fibrinogen (FI)
thrombin also activates FXIII - cross links fibrin clot making it stronger
thrombin activates FVIII and FXI: upregulates cascade
what are the anomalies for the cascade model
high molecular weight kininogen (HMWK), pre-kallikrein and FXII deficiences not associated with bleed, but prolonged APTT
some mammals dont have FXII - not required for normal haemostasis
FXI deficiency (haemophilia C) - variable haemostatic defects
FVIII deficiency (haemophilia A) and FIX deficiency (haemophilia B)- serious bleeding despite intact extrinsic pathway
tissue factor:FVIIIa activates both FIX adn FX
argues against the idea that these pathways are independent generators of FXa
how is the endothelium involved in primary haemostasis
tissue injury leads to vascular spasm of the smooth muscle-localised to area of damage
release of endothelins: vasoconstriction
vasoconstriction slows/stops blood flow-tourniquet effect
small vessel damage my be plugged
larger vessels may require platelets and coagulation factors
Pro-coagulant molecules expressed/ revealed: eg collogen
tissue factor exposed: generates small smount of thrombin
von Willebrand factor multimers released
PRO-COAGULANT FACTORS> anti-coagulant factors
what are the three main phases in platelet plug formation
Adhesion
activation
aggregation
what happens in platelet adhesion
initial step: adhesion to extracellular matrix (collagen)
platelets roll, adhere and spread on collagen matrix-activated monolayer
mediated by interactions between:
vWF adn glycoprotein (GP) Ib/V/IX receptor (high shear blood flow)
collagen and GPVI (major): activation and de-granulation
what happens with platelet activation
multiple pathways of activation: collagen adenosine diphosphate (ADP) thromboxane A2 (TXA2) adrenaline serotonin
thrombin most potent activator: extremely low levels required
multiple effects:
platelet shpe change (stellate)
pro-inflammatory molecule expression: p-selectin
procoagulant activity
degranulation: ADP/TXA2 secretion
converts GP IIb/IIIa into active form: aggregation
Leads to recruitment and activation of platelets to growing stable plug
what happens in platelet aggregation
GPIIb/IIIa central to platelet aggregation
promotes: adhesion aggregation spreading of monolayer thrombus formation/ stability-fibrinogen bound to GPIIb/IIIa bridges
what are the three main phases of the cell based model
initiation
amplification
propagation
what is the initiation phase of the cell based model
(tissue injury)
TF expressing cell rapidly binds FVIIa 1:1 complex
additional FVII activated: upregulated cascade
small smounts of FIX and FX activated
FXa directly but slowly activates FV
FXa binds to its cofactor FVa and its substrate FII:
prothrombinase complex
generation of small amounts of thrombin
what is the amplification phase of the cell based model
the small amount of thrombin produced is pivotal to amplification
activates platelets: via FV adn FXI on platelet surface
cleaves and activates FVIII from vWF: platelet effects
amplification facilitates the availability of important co-factors: FVa and FVIIIa
increased activated platelets results in abundant phospholipid surface for coagulation to take place
what is the propogation phase of the cell based model
platelet granule release: platelet recruitment
FIXa (initiation) binds to FVIIIa (amplification)
additional FIXa generated by FXIa (amplification)
FIXa binds to its cofactor FVIIIa and its sunstrate FX:
intrinsic tenase complex
FXa rapidly binds FVa (amplification) and FII:
thrombin burst
What is the thrombin pivitol to?
procoagulant ‘V’ anitcoagulant
cleaves fibrinogen into fibrin
activates FV,FVIII and FXI: upregulates cascade
activates FXIII - crosslinks fibrinogen for further clot stabilisation
activates platelets via PAR-1 and PAR-4 receptor
inhibits fibrinolysis via thrombin activatable fibrinolysis inhibitor (TAFI
promotes anticoagulation: activation of protein C via thrombomodulimn
name several naturally occuring inhibitors
antithrombin (AT) - previously known as ATIII
protein C (PC)
protein S (PS)
tissue factor pathway inhibitor (TFPI)
how does antithrombin work
SERPIN-SERine Protease INhibitor
Primary targets are thrombin and FXa
Most potent inhibitor of thrombin
FIXa, FXIa and FXIIa also inhibited
Inhibitory activity thousand fold increased by heparins.
how does protein C work
vitamin k-dependent serine protease
cleaves and inhibits FVa and FVIIIa
activated by a complex formed by thrombin and thrombomodulin- negative feedback
activated Protein C (APC) is inhibited by protein C inhibitor (PAI-3) - a serpin
binds to PAI-1 and inhibits its activity: enhancing fibrinolysis
how does protein S work
Vitamin K –dependent glycoprotein
Cofactor for protein C
Protein S exists in 2 forms – bound and free
About 65% complexed to C4b-binding Protein (C4bBP), and is inactive
Only the free form has activity
APC independent anticoagulant: direct binding FVa, FXa and FVIII.
how does TFPI work
TFPI secreted by the endothelium
TFPI directly inhibits FXa
then the FXa:TFPI complex
interacts and inhibits the FVIIa:TF complex
inhibitation of the extrinsic coagulation pathway
what role does fibrinolysis play in the coagulation cascade
after the formation of the clot
the injured vessel is sealed/ bleeding stops
repair of the injury follows
at the end of the process the sealing clots need to be dissolved
removed occlusion from vessels
prevents tissue hypoxia
this is achieved by fibrinolysis
what role does plaminogen play in fibrinolysis
its the precursor of plasmin
activated by t-PA: endothelial cell secreted serine protease
both plasminogen / t-pa bind to lysine residues on fibrin
t-pa:t1/2 (2-3 minutes) due to its potent inhibitor PAI-1
activated by u-PA: extravascular remodelling and repair
affinity for t-PA increases significantly in presence of fibrin clot
what role does plasmin play in fibrinolysis
active serine protease
major enzyme of fibrinolysis
proteolytiv enzyme- enzymatic degradation of fibrin at over 50 cleavage sites
plasmin inhibitors prevent overactive plasmin
confones lysis to local area
summerise initiation
vascualr endothelium injury and circulating blood cells are perturbed: interaction of plasma derived FVIIa and tissue factor
summerise amplification
thrombin activates platelets, cofactors FVa and FVIIIa on the platelet surface and the FXI on the platelet surface
summerise propogation
results in the production of a significant level of thrombin activity, generation of a stable plug a the site of injury and cessation of blood loss
summerise termination
clotting process is limited to avoid thrombotic occlusion in surrounding normal areas of vasculature
