Hemostatic system
Hemostasis
is the dynamic process of coagulation .It stop bleeding in the areas of
vascular injury .This process involves the interaction of
-cellular component:
- Platelets
- Vascular wall
- Protein component :
- Procoagulants( Coagulation factors )
-Fibrinolytics ( for lysis)
-Anticoagulants (regulation)
Blood vessel injury →vasoconstriction
& exposed endothelium & sub endothelial collagen →stimulation of von
Willebrond factor→ platelets activation & adhesion →activated platelets
induce platelets aggregation .At the same time tissue factor + collagen
&other proteins in the tissue activate the coagulation cascade →formation
of thrombin which has multiple effects on coagulation mechanism :
-feedback
activation of factors 5&8
-conversion of fibrinogen to
fibrin
-activation
of factor 2thus a platelet plug is formed &bleeding stop .If failed then
thrombin activate
-factor 13 which
links fibrin to form a stable thrombus. Then contractile elements within
platelets cause clot retraction
•
Thrombin also activate the anticoagulant system
leading to limitation of clot size .Thrombin activate
•
the fibrinolytic system to cause lyses of the
thrombus resulting in recanalysation of the blood vessel.
•
Normal hemostasis
Components
First limb:
•
Coagulation factors
second
limb:
Cell component (platelets and endothelial
cells)
•
Rgulation
–
Coagulation system
–
Fibrinolysis system
–
Anticoagulation system
Platelet structure:
Platelets are non-nucleated cellular fragments produced by megakaryocytes of bone marrow. Their life span is 7-10 days.
Platelets are non-nucleated cellular fragments produced by megakaryocytes of bone marrow. Their life span is 7-10 days.
Normal platelet count is 150 - 400 x 109/ L.
(150.000-400.000/mm3).
Functions of platelets
a- They stimulate vasoconstriction of the injured vessels.
b- They form a primary hemostatic plug to seal small vascular tears. This plug is formed by platelet adhesion (sticking of the platelets to the subendothelial structures) followed by aggregation (sticking of the platelets to each other).
c- They play an essential role in fibrin clot formation by the production of platelet factor 3 needed in blood clot formation.
Functions of platelets
a- They stimulate vasoconstriction of the injured vessels.
b- They form a primary hemostatic plug to seal small vascular tears. This plug is formed by platelet adhesion (sticking of the platelets to the subendothelial structures) followed by aggregation (sticking of the platelets to each other).
c- They play an essential role in fibrin clot formation by the production of platelet factor 3 needed in blood clot formation.
d- They induce fibrin clot retraction.
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