22.12
Lifecycle of Erythrocytes
Erythrocytes, also known as red blood cells, constantly move through blood capillaries. As a result, they damage their plasma membrane due to the cont…
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.
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Q1: Why do erythrocytes become damaged after circulating in the blood?
Erythrocytes constantly squeeze through blood capillaries, experiencing continuous friction that damages their plasma membrane. After 100 to 120 days of circulation, this repeated mechanical stress causes them to become rigid and fragile, making them unable to function properly and ready for removal from circulation.
Q2: Where are damaged erythrocytes removed from circulation?
Damaged erythrocytes get trapped and fragment while passing through small vessels in the spleen and liver. Resident phagocytic macrophages in these organs engulf the damaged cells and break down their components for recycling or elimination. This process is essential for maintaining healthy blood cell populations.
Q3: What happens to hemoglobin when erythrocytes are destroyed?
Macrophages separate hemoglobin into globin and heme groups. Globin chains are broken down into constituent amino acids for reuse in new proteins. Iron extracted from heme is stored as ferritin and hemosiderin complexes, while the non-iron heme remainder converts to bilirubin for bile production.
Q4: How is iron recycled after erythrocyte destruction?
Iron extracted from heme molecules is stored bound to ferritin and hemosiderin, iron-protein complexes. This stored iron is released into the blood and carried by transferrin protein to bone marrow, where it creates new hemoglobin during erythropoiesis. This recycling process conserves iron for continuous red blood cell production.
Q5: What is the fate of bilirubin after erythrocyte breakdown?
Bilirubin, the non-iron remainder of heme molecules, is collected in the liver where hepatocytes secrete it as part of bile into the small intestine for lipid digestion. Bacteria in the large intestine convert bilirubin into urobilinogen, which is mostly converted to stercobilin for fecal elimination.
Q6: How is urobilinogen eliminated from the body?
Large intestine bacteria convert bilirubin into urobilinogen. Most urobilinogen is converted to stercobilin and eliminated through feces as a waste product. However, a small fraction is excreted as urobilin in urine, providing an alternative excretion pathway for the breakdown products of hemoglobin metabolism.
Q7: What components of erythrocytes are recycled versus eliminated?
Globin chains are broken down into amino acids for protein reuse, and iron is recycled for new hemoglobin synthesis during erythropoiesis. In contrast, the heme's non-iron portion becomes bilirubin, which is converted to urobilinogen and eliminated through feces or urine, representing waste products of erythrocyte breakdown.