Once a red blood cell ruptures, hemoglobin is released into the circulation for processing. This is primarily done by the Kupffer cells of the liver and macrophages in the spleen and bone marrow. These macrophages release iron which is carried by transferrin to the bone marrow where it can be reused for the production of new red blood cells. The remaining porphyrin portion of the hemeglobin molecule is converted to bilirubin by the macrophages. The liver cells (hepatocytes) take up the bilirubin, conjugate it and release it in the bile.