Removal
- Some red blood cells will rupture within the blood vessels (aka, intravascular hemolysis), but most aging cells are removed and destroyed within the spleen (extravascular hemolysis).
- Spleen removes malformed cells via two mechanisms:
- The so-called splenic sieve mechanism traps rigid, unpliable cells in capillaries; cells burst, release contents that macrophage will phagocytose.
- Splenic macrophages detect senescent cells, engulf in phagosome.
- In both cases, red blood cell contents are released in splenic macrophage, and hemoglobin is dismantled:
- Heme oxgenase, a macrophage enzyme, releases iron from hemoglobin;
- Porphyrin portion of hemoglobin is converted to bilirubin, which is later excreted in bile by the liver.
- Globin, a protein substance, is broken into its amino acid components and added to the amino acid pool.
Erythrokinetics
- 1% of erythrocytes, aka, red blood cells, are removed and replaced every day.
- The production of new red blood cells, called erythropoiesis, occurs in the red marrow of spongy bone.
- Regulated by hypoxia, sensed by kidney.
- On average, erythrocytes travel in the circulatory system for approximately 120 days; recall that they are responsible for oxygen and carbon dioxide transport.
- Once they are no longer viable, splenic macrophages remove them via phagocytosis.