Haemostasis Flashcards
what are the functions of haemostasis?
To maintain blood in a fluid state while it remains circulating within the vascular system
To confine circulating blood to the vascular bed
To arrest bleeding at the site of injury or blood loss by formation of a haemostatic plug once healing is complete
what are the components of normal haemostasis?
- Blood vessels or vascular system
- Platelets
- Plasma coagulation factors
- Plasma coagulation inhibitors (natural anticoagulants)
- Fibrinolytic system
In the haemostatic state what prevents blood from clotting invivo in the body?
vascular endothelial cells and some of the substances present on these cells including thrombomodulin and heparan sulphate
what do thrombomodulin and heparan sulphate promote?
an anticoagulation environment and enhance the effect of the natural anticoagulant antithrombin, protein c and protein s
example of a vasoconstrictor?
angiotensin II
what does damage to the endothelium and exposure of sub endothelial structures promote?
platelet adhesion with subsequent platelet activation
what do endothelial cells produce?
vasoconstrictors, vonWilldebrande factor, heparin sulphate, procoagulant and anticoagulants
how are platelets produced?
in the BM by fragmentation of the cytoplasm of megakaryocytes
what is the major regulator of platelet production?
Thrombopoietin the platelet growth factor which is produced by the liver and kidneys
what are stored within the platelet granules?
Important pro and anticoagulant factors and thromboxane A2 TXA2 a powerful vasoconstrictor
where is vWF synthesised and stored?
synthesised both in endothelial cells and megakaryocytes and stored in weibel palade bodies in the endothelial cells and platelet alpha granules respectively
what are the assays that screen for defects in the coagulation factors?
Prothrombin time PT
Activated partial thromboplastin time APTT
Fibrinogen assay
Thrombin time TT
what does the prothrombin time test do?
measures the clotting time of plasma in the presence of an optimal concentration of tissues extract (thromboplastin and calcium) and indicates the overall efficiency of the extrinsic clotting system
how is the prothrombin test carried out?
trisodium citrate sample centrifuged and mixed with thromboplastin with calcium chloride
time to clot measured (9-13 secs)
what are some conditions that cause prolonged bleeding time?
Inherited platelet function defects eg glanzmann’s thrombasthenia, Bernard Soulier syndrome
Vascular abnormalities - ehlers danlos syndrome
Thrombocytopenia.