Haemostasis Flashcards
define haemostasis?
arrest of bleeding from a broken/ruptured blood vessel
- prevents significant blood loss after vascular injury
what does haemostasis depend on?
- blood vessel wall
- platelets + other blood cells
- coagulation proteins/other proteins
haemostasis brief process
- localised vasoconstriction - dec blood flow to injury site + stops blood loss
- primary haemostasis (platelet plug form) - plugs breach in blood vessel
- secondary haemostasis (blood clotting/coagulation) - strengthens + reinforces platelet plug
- tertiary haemostasis (fibrinolysis) - dissolves clot once blood vessel integrity restored
localised vasoconstriction
- injured blood vessel constricts
- mediated by reflex neurogenic mechanisms + released—> vasoconstrictor (e.g. endothelin)
- transient - lasts up to 30 mins
- briefly reduces blood loss to injury site
what does the endothelium act as?
physical barrier
separates circulating platelets from thrombogenic substances in extravascular space
when the procoagulant sub endothelial matrix is exposed, what does it initiate?
PRIMARY HAEMOSTASIS
- platelet adhesion
- platelet activation
- platelet aggregation
- platelet plug formation
platelet adhesion
- on endothelial injury, platelets bind —> exposed subendothelial matrix proteins (e.g. collagen)
- bind via transmembrane glycoprotein receptors —>
> GP1b-IX-V binds von willebrand factor
GPVI + alpha-2-beta-1 (GPIa-IIa) binds directly to collagen
GP1c/IIa binds fibronectin
platelet activation
UNDERGOES CHANGES
shape change - discoid shape —> elongated cells with cytoplasmic extensions
granule release -
> alpha granules: vwf, p-selection, FV + FXIII
> dense granules: ADP, serotonin, ca2+
membrane phospholipid metabolism —> inc thromboxane A2 (TXA2) prod
activation + expression of GPIIb/IIIa receptors
platelet aggregation + plug formation
- agonist activated platelet GPIIb/IIIa receptors bind fibrinogen
- —> crossbridges with platelets next to
- —> formation of primary haemostatic plug
plug only good for haemorrhage in small blood vessels
activated + aggregated platelets form. phospholipid membrane for secondary process
what does major activation of secondary haemostasis need?
- neg charged phospholipid-rich membrane surface (on activated platelets, endothelial cells..)
- enzyme coat factor, substrate, cofactor, ca2+
- 3 pathways: intrinsic, extrinsic + common
clotting factors of secondary haemostasis?
- serial activation of inactive plasma proteins (alpha + beta golublins) —> active enzymes + cofactors
- activated enzyme + inactive plasma protein —> active enzyme
- happens until fibrinogen —> fibrin by thrombin
- all enzymes = serine proteases
most factors synthesised in liver
classification of coagulation factors
state the coag. factor and properties of the contact group
XII, XI prekallikrein, HMW kininogen
needs contact with negative charged surface for activation
classification of coagulation factors
state the coag. factor and properties of the prothrombin group?
II, VII, IX, X
needs vitamin K for synthesis
classification of coagulation factors
state the coag. factor and properties of the fibrinogen group?
I, V, VIII, XIII
large molecules absent from serum
coagulation pathways
- intrinsic —> all components occur in blood
- extrinsic —> needs factor not in blood stream
both activate common pathway
intrinsic
SLOW (when blood in contact with collagen in damaged blood vessel)
- PK
- HMW K
- factor XII, XI, IX, VIII
- FXII (hageman factor) = primary complex with HMWK + PK on collagen —> activation of FXII
- FXIIa activates FXI -> activates -> FIX
- FIXa = complex with FVIIa, ca2+ + phosphatidylserine (intrinsic tenase)
- —> activates FX (common pathway) —> thrombin gen
extrinsic
FAST (traumatic injury)
- interaction between TF, FVII and ca2+
TF (FIII, tissue thromboplastin)
- cell bound glycoprotein
- in extravascular tissues e.g. fibroblasts
- exposed to blood when endothelial barrier breached
- contact with blood, extravascular cells release TF
- TF forms complex with FVII + ca2+ (extrinsic tenase)
- TF-FVIIa complex activates FX of common pathway —> thrombin gen)
can activated FX on intrinsic pathway (alternative pathway)
common
- factor X , V
- factor II ( prothrombin)
- factor I (converts fibrinogen —> fibrin)
- activation of FX
- conversion of prothrombin (FII) to thrombin (FIIa)
- cleavage of fibrinogen (FI) to fibrin (FIa)
- polymerisation of fibrin
- stabilisation of fibrin polymers by FXIII
what are the limitations of the coagulation cascade model?
- doesn’t fully explain how blood clots in vivo
- assumes intrinsic + extrinsic pathways independently capable of initiating clot form
- deficiency of FXII, HMWK, PK doesn’t lead to bleeding tendency
- FVIII/IX deficiency —> serious bleeding tendency. even through extrinsic pathway intact
- FVII deficiency —> serious bleeding tendency. even though intrinsic pathway intact
modern cell based model
secondary haemostasis happens in different overlapping steps
- initiation
- amplification
- propagation
needs 2 cell types:
- TF-bearing cells
- platelets ———> kept apart until vascular injury
modern cell based model
initiation
- TF released from injured cells + monocytes
- TF + FVIIa —> TF/FVIIa complex
- TF/FVIIa activates small amount FIX + FX —> generates small amount thrombin
- FXII have minor role in activation of FXI
modern cell based model
amplification
- thrombin activates FV to Va
- FVIII —> FVIIIa + activates more platelets
- also activates FXI to FXIa
modern cell based model
propagation
- FXa produced when TF/FVIIa complex activates FIX
- FIXa + FVIIIa —> tenase complex
- —> converts more FX to FXa
- FXa + FVa + ca + phoshoplipid (PL) surface (activated platelets) form prothrombinase complex
- turns prothrombin —> thrombin (large amounts)
tertiary haemostasis
fibrinolysis
- ensures localisation of fibrin clot formation + clot removal
- carried out by plasmin (proteolytic enzyme)
- degrades fibrin mesh
- release of plasminogen activators (t-PA, FXIIIa, v-PA, kallikrein)
- plasmin produced from inactive precursor, plasminogen
- clot lysis + release of degradation products
regulation of haemostasis
- primary + secondary haemostasis restricted to local site of vascular injury
- size of primary + secondary plugs restricted —> keep blood vessels patent
- fibrinolytic system regulated —> ensure removal of unwanted fibrin clots + preservation of fibrin
- regulation achieved via ENDOGENOUS antithrombotic + antifibrinolytic systems
endothelium mechanism of action
global haemostasis: physical barrier
primary haemostasis: secrete platelet antagonists (nitric oxide, ADPase, prostacyclin)
secondary haemostasis: expresses thromobmodulin
- binds thrombin + activates protein C (inhibits co-factors FVa, FVIIa) + endothelial protein c receptor
- expresses surface heparin-like (glycosaminoglycans)
—> enhances AT and TFPI activity
fibrinolysis: secrete fibrinolytic inhibit e.g. plasminogen activation inhibitor - 1 and 2
plasma proteins (protease inhibitors) mechanism of action
global haemostasis: produced in liver
e.g. alpha2 - macroglobulin
alpha1 - antitrypsin
acts as non-specific inhibition of serum proteases
regulators of secondary haemostasis
tissue factor pathway inhibitor
- made in endothelial cells
- inhibits TF-FVIIa + TF-FVIIa-FXa complex
- inhibits free factor Xa generation by TF-FVIIa complex (initiation of secondary haemostasis)
regulators of secondary haemostasis
antithrombin III (AT III)
- made in liver
- inhibits FIIa, FXa, FXIIa, FIXa, FXIa, TF-FVIIa
- activity enhanced by heparin (exogenous) + heparin-like GAGs on endothelial cells
regulators of secondary haemostasis
protein c
- vitamin k dependent
- made in liver
- plasma glycoprotein
- inactivates FVa, FVIIa
- activated by thrombin via binding to thrombomodulin on endo. cells
- further activated by binding of thrombomodulin-thrombin complex to endothelial protein c receptor
regulators of secondary haemostasis
protein s
- vitamin k dependent
- made in liver
- cofactor for TFPI + activated protein c
- exists as free/bound to acute phase protein - C4 finding protein
ONLY FREE PROTEIN S
regulators of fibrinolysis
thrombin activatable fibrinolytic inhibitor (TAFI)
- carboxypeptidase B
- made in liver
- binds to lysine residues in fibrin —> prevents plasminogen binding + activation
- activated by thrombin burst (amp + prop of 2 haem)
regulators of fibrinolysis
plasminogen activator inhibitor 1 (PAI-1)
- made by endothelial cells
- binds + inactivates tissue plasminogen activator
- inhibited by thrombin-thrombomodulin complex
regulators of fibrinolysis
alpha2- antiplasmin
- binds + inactivates plasmin
- made in liver
regulators of fibrinolysis
polyphosphates
- released from dense granules in platelets during activation
- forms dense fibrin network —> resists lysis
regulators of fibrinolysis
extracellular nuclear material (histones, DNA…)
- binds to fibrin network
- inhibits fibrin degradation
- promotes PAI-1 inhibition of tPA
