Tracking neutrophils from bone marrow to blood, modeling using built-in components in SAAM II. (A) Model describing the flux of labeled neutrophils through the hematopoiesis system, using built-in components in SAAM II. Distribution of labeled neutrophils between marrow and blood assuming blood half-lives of (B) 3.5 days and (C) 7.5 hours. In both cases, the model fits the specific activity in the plasma neutrophils to published specific activity measurements for thymidine-tagged neutrophils in circulation.²