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How many platelets are required for normal hemostasis

- >50,000 platelets

Platelet function in Primary Hemostasis

-form platelet plug for initial arrest of bleeding

Platelet function in Secondary Hemostasis

-provide binding sites for coagulation proteins invovled in the formation of fibrin
-interact with fibrin strands to cause clot retraction: clot shrinks as serum is squeezed from the fibrin mass, wound is sealed preventing further blood loss
-clot contraction permits return of a more normal blood flow through the vessel

Platelets play a role in both

-the formation of a primary plug as well as the coagulation cascade

Endothelial Cells

-line the lumen of the vessel

A break in the lumen will

-intiate a series of platelet reactions (primary hemostasis)


-platelets bind to non-platelet surfaces such as:
-subendothelial collagen
-von Willebrand factor (dependent on this)

Adhesion Characteristics

-when endothelial cells are damaged, platelets in the bloodstream are exposed to underlying collagen
-this contact inititates platelet adhesion in which platelets stick to the damaged area of the blood vessel to form a loose clot


-platelets adhere to each other
-process is irreverisible

Aggregation Characteristics

-in response to chemical changes in the platelets, platelets stick to other platelets to form a platelet plug
-process is initiated and requires the presence of membrane glycoprotein, fibrinogen-binding sites, fibrinogen and calcium
-platelets undergo shape change from disc to spiny spheres and stick to injured tissue

Platelet Secretion

-platelets release the contents of their dense granules and bodies
-process is irreversible

Platelet Secretion Characteristics

-granules release constitutes a secondary aggregation which is irreversible
-ADP, Serotonin, and Calcium are secreted
-theses components facilitate and regulate clot formation
-secretion can only occur after sufficiently strong platelet stimulus that results in an irreversible process
-platelet plug is stabilized through the formation and deposit of fibrin

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