Platelets are derived from megakaryocytes, the largest bone marrow hematopoietic cells. At any given time, two-thirds of the total platelets are found within the circulation while one-third are sequestered within the spleen. In persons with enlarged spleens, 80-90% of the platelets are in the spleen, resulting in a decreased concentration of circulating platelets. In individuals who have had a splenectomy, all of the platelets will be in the circulating blood. The life span of the platelet is 8-10 days.