The Roles of Platelets

1. Primary aggregation. Platelets in the damaged region attach to collagen revealed by the discontinuity in the vessel wall, forming a platelet plug.

2. Secondary aggregation. Platelets in the plug release the contents of their alpha and delta granules. This release of serotonin explains the higher concentration of serotonin in serum than in plasma. Scrotonin, a vasoconstrictor, restricts blood flow to the damaged area by causing contraction of vascular smooth muscle.

3. Blood coagulation. Platelets release fibrinogen in addition to that normally found in the plasma. The fibrinogen is converted by the clotting factor cascade into fibrin, which forms a dense fibrous mat to which more platelets and other blood cells attach, forming a clot and plugging the opening in the blood vessel wall.

D. Clot Retraction: The clot initially bulges into the vessel lumen, but later it contracts and condenses through the interactions of thromhosthenin and platelet actin, myosin, and ATP.

E. Clot Removal: As the vessel wall heals and the protection afforded by the clot is no longer needed, the clot is removed by the enzyme plasmin, Plasmin is formed by the action of plasminogen activators on the plasma proenzyme plasminogen Enzymes released by the lambda granules (lysosomes) of the platelets also aid in clot digestion