Platelets are irregular-shaped, membrane-enclosed cellular fragments that do not contain a nucleus. Platelets are continually produced in the red bone marrow. They serve an important function in blood clotting. Normally, the concentration of platelets in an adult ranges from 150,000 to 450,000 per microliter, although the count may rise further during times of stress. Platelets can circulate in the blood for about 8-9 days, unless they are needed earlier for blood clotting. An abnormally small number of platelets in circulating blood is termed thrombocytopenia.