Lifecycle of Erythrocytes

The circulating erythrocytes frequently squeeze through blood capillaries, damaging their plasma membrane due to constant friction.

After about 100 to 120 days, erythrocytes wear out, becoming rigid and fragile. They get trapped and fragment while passing through the small vessels in the spleen and liver.

Here, the resident phagocytic macrophages engulf these damaged cells, separating their globin and heme groups.

The globin chains are broken down into constituent amino acids for reuse.

Meanwhile, the extracted iron from the heme molecules is stored bound to ferritin and hemosiderin — iron–protein complexes.

The stored iron is released into the blood and carried by a transferrin protein to create new hemoglobin during erythropoiesis in the bone marrow.

The non-iron remainder of the heme molecules is eventually converted to bilirubin and collected in the liver.

Hepatocytes secrete bilirubin as part of bile into the small intestine for lipid digestion.

Lastly, the large intestine bacteria convert the bilirubin into urobilinogen.

While most urobilinogen is converted to stercobilin for fecal elimination, a small fraction is excreted as urobilin in urine.