Haemostasis Flashcards
What is haemostasis?
the arrest of bleeding and maintenance of vascular patency
What are the components of haemostasis?
platelet plug formation
fibrin clot formation
fibrinolysis
anticoagulant defences
What is primary haemostasis?
platelet plug formation
What is secondary haemostasis?
fibrin clot formation
How are platelets formed?
in bone marrow by ‘budding’ from megakaryocytes
What is the life span of platelets?
7-10 days
Describe primary haemostasis.
1 - endothelial damage exposes collagen
2 - releases von willebrand factor (and other proteins) to which platelets have receptors
3 - platelet adhesion to site of injury
4 - secretion of prothrombotic chemicals –> aggregation of platelets at site of injury, forming platelet plug
What can cause failure of the platelet plug?
vascular problems
vWF problems
platelet problems - reduced number or function
What are the consequences of failure of the platelet plug?
spontaneous bruising and purpura
retinal haemorrhage
mucosal bleeding - petechiae
What tests can be used to screen primary haemostasis?
platelet count
What can cause failure of the fibrin clot?
increased fibrinolysis
single clotting factor deficiency e.g. haemophilia
multiple clotting factor deficiencies e.g DIC
What activates formation of the fibrin clot?
TF (tissue factor)
What is fibrin?
insoluble, cross linked mesh stabilising platelet plug
Describe fibrinolysis.
tissue plasminogen activator released from endothelial cells (stimulated by fibrin formation)
cleaves plasminogen to plasmin
plasmin causes fibrin to break down into FDPs
What are FDPs?
fibrin degeneration products
What are screening tests for secondary haemostasis?
prothrombin time
activated partial thromboplastin time (APTT)
What are the consequences of failure of fibrin formation?
pattern of bleeding depends on clotting factors involved and whether its single or multiple abnormalities
What are the naturally occurring anticoagulants?
serine protease inhibitors e.g. antithrombin III
protein C and protein S
What are thrombophilias?
increased tendency to develop VT
happen when there is a deficiency of naturally occurring anticoagulants
Describe the extrinsic and intrinsic pathways of secondary haemostasis.
extrinsic pathway: TF activates factor VII activates factor X and V cleaves prothrombin to thrombin cleaves fibrinogen to fibrin
intrinsic pathway:
series of clotting factors activates factor IX and VIII
these help propagate the cascade (amplify activation of X and V)
Which important clotting factors are in the extrinsic pathway?
TF (factor II)
V
VII
X
Which important factors are in the intrinsic pathway?
IX
VIII
