Erythrocytes, also known as red blood cells, constantly move through blood capillaries. As a result, they damage their plasma membrane due to the continuous friction. Typically, after 100 to 120 days, erythrocytes become rigid and fragile as they wear out. As they pass through small vessels in the spleen and liver, they can get trapped and break apart into fragments.

The resident phagocytic macrophages deal with these damaged cells by engulfing them and separating their globin and heme groups. The globin chains are then broken down into amino acids that can be reused. Meanwhile, the iron extracted from the heme molecules is stored as iron-protein complexes known as ferritin and hemosiderin. This stored iron is then released into the blood and carried by a transferrin protein to create new hemoglobin during erythropoiesis, which occurs in the bone marrow.

The non-iron remainder of the heme molecules eventually converts to bilirubin, which is collected in the liver. Hepatocytes secrete bilirubin as part of bile into the small intestine for lipid digestion. Lastly, bacteria in the large intestine convert bilirubin into urobilinogen. Although most urobilinogen is converted to stercobilin and eliminated through feces, a small fraction is excreted as urobilin in urine.