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Hemostasis

Hemostasis

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Three Key Events:
1. Vascular Spasm: vasoconstriction upon blood vessel damage, narrows luminal diameter 2. Platelet Plug Formation & Coagulation: Platelet Plug Formation:* accumulation of platelets in damaged vessel walls. Coagulation:* conversion of liquid blood into a gel (clot).
Vascular Spasm
  • Initiated by exposed collagen – slows flow of blood to minimize blood loss.
Platelet Plug Formation 1. Adhesion: wherein platelets tether to the exposed collagen 2. Activation: initiated by adhesion, produces intracellular rearrangement of action that results in spiky processes on platelets (star-shape). 3. Aggregation: positive feed back loops.
  • Activated platelets release ADP and thromboxane A2.
– ADP makes other platelets stickier. – Thromboxane A2 stimulates ADP release.
Coagulation
  • Intrinsic: coagulation factors in the blood trigger cascade.
  • Extrinsic: coagulation factors released by tissues enter blood stream and
trigger cascade. Extrinsic pathway is shorter than intrinsic pathway; they converge at factor X activation
Shared pathway:
  • Activation of Factor X (halfway through intrinsic/beginning of extrinsic)
– Active Factor X + Factor V + calcium ? Prothrombinase – Prothrombinase + Prothrombin (Factor II) ? Thrombin – Thrombin + Fibrinogen (Factor I) ? Fibrin (loose mesh) – Fibrin (loose mesh) ? Fibrin (strong mesh)
  • Fibrin meshwork forms over platelet plug ? traps red blood cells and aggregating platelets.

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