Haemostasis Flashcards
what is haemostasis
process of blood clot formation at the site of vessel injury
what is the response of a vessel to injury
vasoconstriction – reduced blood flow to the injured area
what happens in primary haemostasis
formation of the platelet plug
what happens in secondary haemostasis
formation of the fibrin clot
components of normal haemostatic system
formation of platelet plug (primary)
formation of fibrin clot (secondary)
fibrinolysis
anticoagulant defences
how are platelets formed
budding from megakaryocytes in the bone marrow
platelet lifespan
7-10 days
how is the platelet plug formed
endothelial (vessel wall) damage exposes collagen and releases Von Willebrand Factor (VWF)
Platelet membrane receptor glycoprotein Ib (Gplb) has VWF receptors
Platelet adhesion at the site of injury
there is then secretion of chemicals from the platelets which leads to aggregation of platelets at the site of injury
what do platelets release at the site of injury
ADP
serotonin
thromboxane A2
fibrinogen
why might formation of a platelet plug fail
vascular
- loss of collagen in vessel walls makes it easier for them to break down
platelets
- reduced numbers
- reduced function
VWF
- reduced levels or not functioning
consequences of failure platelet plug formation
spontaneous bruising + purpura
mucosal bleeding
- epistaxis
- GI
- Conjunctival
- menorrhagia
intracranial haemorrhage
retinal haemorrhage
when is menorrhagia a key presenting complaint
von willebrand problems
screening test for primary haemostasis
platelet count
what 3 things are needed for formation of a platelet plug
platelets
VWF
collagen in vessel wall
how is a fibrin clot formed
- tissue factor is released from damaged vessel walls
this binds the first clotting factor – factor 7 - this then activates clotting factors 5 and 10
- factors 5 and 10 turn prothrombin into thrombin
- thrombin then turns fibrinogen into fibrin
- thrombin also activates fact 8 and 9
- they then activate more clotting factor 5 and 10
- feedback loop established
what step in fibrin clot formation is called initiation
when TF is released and activates factor 7
what step in fibrin clot formation is called is called propagation
clotting factor 5 and 10 converting prothrombin into thrombin
what step in fibrin clot formation is called amplification
thrombin activation factor 8 and 9 which go on to activate more 5 and 10
causes of fibrin clot failure
single clotting factor deficiency
- e.g. haemophilia (factor 8 deficiency)
multiple clotting factor deficiency
- e.g. disseminated intravascular coagulation
increased fibrinolysis
- part of complex coagulopathy
how does fibrinolysis occur
plasminogen converted to plasmin by tissue plasminogen activator (tPA)
plasmin converts fibrin to fibrin degradation products (FDP)
how is FDP measured
d-Dimers
screening tests for fibrin clot formation
prothrombin time (PT)
activated partial thromboplastin time (APTT)
what does prolonged PT suggest
problem with TF or factor 7
what drug can increase PT time
warfarin
what does prolonged APTT suggest
problem with factor 8 or 9
what drug can increase APTT time
heparin
how does anti-coagulation occur
anti-thrombin 3 binds to thrombin + switched off TF, factor 7, factor 5 and 10, factor 8 and 9
what is protein C and protein S
co factors activated by thrombin that switched off factor 5, 10 and 8
