Haemostasis- platelet Flashcards
what is haemostasis?
the arrest of bleeding and the maintenance of vascular patency
what is primary haemostasis?
-formation of a platelet plug
what is secondary haemostasis?
-formation of fibrin clot
would a paper cut use primary or secondary haemostasis?
primary haemostasis (formation of platelet plug
what are the components of a normal haemostatic system?
- Formation of platelet plug - primary haemostasis
- Formation of fibrin clot - secondary haemostasis
- Fibrinolysis
- Anticoagulant defences
where are platelets formed and what from ?
in bone marrow from ‘budding’ megakaryocytes
average life span of a platelet?
7-10 days
how is the life span of a platelet clinically relevant?
Aspirin will affect platelet for its whole life span (life span is 7-10 days)
-Stop aspirin 7 days before operation so patient wont bleed as there will be new platelets
structure of a platelet?
-platelets are small anucleated discs
role of Von Williebrand Factor (VWF)?
Von Williebrand factor helps platelets stick together AND binds to factor 8 (so if have defect, this factor will fall)
explain primary haemostasis?
-Endothelial (vessel wall) damage exposes collagen and releases Von Willebrand Factor (VWF), and other proteins to which platelets have receptors causing platelet adhesion to site of injury
-There is then secretion of various chemicals from the platelets, which leads to aggregation of platelets at the site of injury
Von Willebrand Factor: helps with platelet adhesion AND binds to factor 8
what could cause failure for platelet plug formation?
Causes: Vascular, Platelets, Von Willebrand Factor
Vascular no collagen can lead to bleeding
* Hereditary: Marfans
* Acquired: Vit C deficiency (scurvy), HSP vasculitis, increase in age
Platelets
* Reduced number (thrombocytopenia) - majority are aquired
-Reduced production (marrow problem)
-Increased destruction (coagulopathy) e.g. DIC, autoimmune e.g. immune throbocytopenic purpura, hypersplenism
* Reduced function -majority are Aquired -Drugs: aspirin, NSAIDs, renal failure
Vol Willebrand Factor autosomal dominant, most common haemolytic disease
vascular causes of failure for platelet plug formation?
Vascular (no collagen can lead to bleeding)
-Hereditary: Marfans
-Acquired: Vit C deficiency (scurvy), HSP vasculitis, increase in age
platelet causes of failure for platelet plug formation?
Can either be due to reduced number of platelets (either due to reduced production or increased destruction) or reduced function
Reduced production examples:
-bone marrow suppression
Increased destruction examples:
-DIC, immune thrombocytopenic purpura
Reduced function examples:
-drugs such as aspirin, renal failure or von willebrand factor disorder
consequences of failure to form a platelet plug?
-Spontaneous Bruising and Purpura
-Retinal haemorrhages
-Mucosal bleeding:
* Epistaxes
* Gastrointestinal
* Conjunctival
* Menorrhagia
* Intracranial haemorrhage
what are screening tests for primary haemostasis?
Check platelet count
(no simple screening tests for other components of primary haemostasis)
describe the coagulation pathway
Platelets are full of -ve phospholipid, when platelets are activated, +ve charge calcium is recruited and binds to platelets phospholipid. Blood clotting factors are -ve charged and so are attracted
-when endothelial damage TF binds to clotting factors VIIa
-VIIa and TF activate V and Xa
-V/Xa cause II (prothrombin) to become IIa (thrombin)
-IIa (thrombin) causes (I) fibrinogen to become (Ia) fibrin
-also IIa (thrombin) can activate VIIIa and IXa which activate more V and Xa creating a positive feedback loop
what is the intrinsic coagulation pathway?
clotting factors VIII (8) and IXa (9a)
what is the extrinsic coagulation pathway?
TF and VIIa
sEvEn= Extrinsic
what clotting factor plays an important role in combining the pathways?
Xa
X marks the spot
how do platelets bind to subendothelial collagen?
Platelets bind to subendothelial collagen via Glycoprotein 1b and Von Willebrand Factor.
what can measure the extrinsic pathway?
prothrombin time
what clotting factors are affected- increased PT and normal APTT?
clotting factor VII
what clotting factors are affected- normal PT and increased APTT?
Clotting factors:
VIII
IX
XI
XII
