Platelets (or thrombocytes ), are the smallest of the cells in the blood. Platelets, which are produced in the bone marrow, are disk-shaped and have no hemoglobin. Thrombopoietin, a hormone mainly produced by the liver, stimulates platelet production. If the platelet count is decreased, more thrombopoeitin circulates freely and increases marrow production. If normal amounts of platelets, calcium and tissue factors are present, a chain reaction of clotting factors (prothrombin, thrombin, and fibrinogen ) produce a substance called fibrin. Fibrin forms a mesh over the wound in which all of the formed elements are kept rigid. Clotting can also occur in abnormal situations within the blood vessel, forming an embolus or thrombus.
A normal platelet count in a healthy person is between 150 and 400 (x 10 9 /L of blood). Generally, low platelet counts increase bleeding risks. Low platelet counts are generally not corrected by transfusion unless the patient is bleeding or the count has fallen below 5.