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Key Cardiovascular Variables and Their Normal Determinants
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Whenever damage occurs to a blood vessel, a variety of processes are evoked that are aimed at preventing or stopping blood from exiting the vascular space. The 3 primary processes are summarized in the following list:
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Platelet aggregation and plug formation: Occur as a result of the following steps:
Vessel injury with endothelial damage and collagen exposure.
Platelet adherence to collagen (mediated by the plasma protein, von Willebrand factor).
Platelet shape change (from disks to spiny spheres) and degranulation with release of the following:
Adenosine diphosphate, which causes platelet aggregation and “plugs” the hole.
Thromboxane, which causes vasoconstriction and potentiates platelet adhesion and aggregation.
Local vasoconstriction: Mediated largely by thromboxane but may also be induced by local release of other chemical signals that constrict local vessels and reduce blood flow.
Blood clotting: The formation of a solid gel made up of the protein, fibrin, platelets, and trapped blood cells.
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The critical step in blood clotting is the formation of thrombin from prothrombin, which then catalyzes the conversion of fibrinogen to fibrin. The final clot is stabilized by covalent cross-linkages between fibrin strands catalyzed by factor XIIIa (the formation of which is catalyzed by thrombin).
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