Role of platelets in hemostasis and thrombosis.

Platelets are central to both normal hemostasis and abnormal thrombotic states along with the vessel wall, coagulation elements, and blood flow. The platelets play a pivotal role in the reaction that occurs after vessel injury, during which platelets first adhere to the vessel wall, undergo a release reaction and then aggregate, probably as a result of the materials released from platelets. These processes can be studied by a series of in vitro tests which form the basis of our knowledge of platelets in hemostasis. While the hemostatic plug is usually microscopic in size, this same plug (platelet thrombus) may contribute to the pathogenesis of several arterial diseases such as transient ischemic attacks, sudden blindness, sudden cardiac death and acute respiratory death syndrome. Careful microscopic examinations have shown that platelet aggregates may be found in the microcirculation which could affect vital structures such as the conduction system of the heart. Both anatomic and therapeutic evidence evidence suggests that platelets play a role in venous thrombosis. Recent evidence suggests increased levels of materials known to be released from platelets in patients with both arterial and venous thrombi along with increased platelet coagulant activities in patients with venous thrombosis.