Platelet Function

How many platelets required?

>50,000 platelets are required for normal hemostasis Platelets participate in Primary and Secondary

Primary Hemostasis

Form platelet plug for initial arrest of bleeding

Platelet Secretion Characteristic

Granule release constitutes a secondary aggregation which is irreversible. The following are secreted during a platelet secretion: ADP, Serotonin, and Calcium. These components facilitate and regulate clot formation. Secretion can only occur after sufficiently strong platelet stimulus that results in an irreversible process. The platelet plug is stabilized through the formation and deposit of fibrin.

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. During aggregation, platelets undergo shape change from disc to shiny spheres and stick to injured tissue.

Aggregation Definition

Platelets adhere to each other. This process is irreversible.

Adhesion Definition

Platelets bind to non-platelet surfaces such as -Subendothelial collagen -von Willebrand Factor

Platelets Role in Hemostasis

Platelets play in both the formation of a primary plug as well as the Coagulation Cascade. The lumen of the vessel is lined with endothelial cells. A break in the lumen will initiate a series of platelet reactions (Primary)

Platelet Secretion Definition

Platelets release their contents of their dense granules and bodies. The process is irreversible.

Secondary Hemostasis

Provide binding sites for coagulation proteins involved 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.

Adhesion Characteristics

When endothelial cells are damaged, platelets in the bloodstream are exposed to underlying collagen This contact initiates platelet adhesion in which platelets stick to the damaged area of the blood vessel to form a loose clot. Adhesion is dependent upon von Willebrand Factor