Haemostasis and Blood Coagulation
Haemostasis the physiologic process that stops bleeding – it is reliant on the co-ordinated action of vascular factors, platelet factors and coagulation factors.
Three events/phases of haemostasis – vascular spasm, formation of platelet plug and clotting of the blood (coagulation).
Vascular spasm:
Blood vessels undergo immediate vasoconstriction following damage
Platelet plug formation:
Normally platelets do not adhere to each other or to vessel wall, but when collagen fibres are exposed by damage platelets adhere to each other and exposed collagen. Platelets become activated aggregation (complex process that builds fibrinogen links between adjacent platelets) “plug” of platelets to stop bleeding
Coagulation:
3 phase process that clots blood:
1) Prothrombin activator is formed
2) Prothrombin activator converts prothrombin to thrombin
3) Thrombin catalyses fibrinogen molecules fibrin mesh to trap blood cells
Two pathways mediate this process:
1) Intrinsic (all clotting factors are present in vascular system)
2) Extrinsic (requires thromboplastin from outside the vascular system)
Coagulation Factors (12):
I Fibrinogen
II Prothrombin
III Tissue factor
IV Calcium
V Proaccelerin
VII Proconvertin
VIII Antihaemophilic factor
IX Plasma thromboplastin component
X Stuart-Prower factor
XI Plasma thromboplastin antecedent
XII Hageman factor
XIII Fibrin stabilising factor
Clot retraction and repair:
Platelets contain actinomysin contracts to draw edges of vessel together and serum squeezed out repaired by smooth muscle and connective tissue and endothelial cell migration
Fibrinolysis:
Digestion of clot
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