Haemostasis and Blood Coagulation

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