Describe coagulation and the role that platelets play in the process.
Clotting can be thought of as occurring in two phases: primary and secondary hemostasis. Primary hemostasis is the creation of the initial platelet plug and secondary hemo-stasis is the creation of the stable fibrin clot. Platelets play an important role in both processes. Generally, blood vessels prevent platelet adhesion by releasing a potent vasodilator, prostacyclin. After vessel wall injury, prostacy-clin levels decrease and platelets adhere to the vessel wall. Adhesion leads to activation and degranulation with release of adenosine-5-diphosphate (ADP), serotonin and thromboxane that leads to platelet aggregation. Further aggregation leads to formation of a platelet plug. This plug is unstable and requires fibrin formation, which occurs by activation of the intrinsic and/or extrinsic coagulation sys-tem. Platelets provide the phospholipid membrane upon which the coagulation cascade occurs. Platelet abnormali-ties can be qualitative or quantitative and are the most common hematologic disorders during pregnancy.