Because they lack cellular organelles and thus the physiology to maintain themselves

Because they lack cellular organelles and thus the physiology to maintain themselves

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Because they lack cellular organelles and thus the physiology to maintain themselves,  erythrocytes survive for only about 120 days. Degenerated erythrocytes are broken  down in the spleen and liver by macrophages (phagocytic white blood cells) as follows:  1. The globin and heme parts of the hemoglobin are separated. The globin is  reduced to amino acids, which are returned to the blood plasma. 2. Iron is removed from the heme group and bound to the proteins ferritin and  hemosiderin, which store the iron for later use (because unbound iron is toxic).  Iron is also attached to transferrin, which enters the bloodstream. Transferrin may  be picked up by muscles or liver cells, where it may be stored as ferritin or  hemosiderin or picked up by bone marrow, where the iron is used to produce new 
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Unformatted text preview: erythrocytes. 3. The remainder of the heme group is broken down into bilirubin (a yellow-orange pigment), which enters the bloodstream and is picked up by the liver. Liver cells incorporate bilirubin into bile, which enters the small intestine during the digestion of fats. Bilirubin is then converted into urobilinogen by intestinal bacteria. Finally, most urobilinogen is converted to the brown pigment stercobilin, which is eliminated with the feces (and which gives feces its brown color). A small amount of urobilinogen is absorbed into the blood, converted to the yellow pigment urobilin, picked up by the kidneys, and eliminated with the urine (contributing to the yellow color of urine)....
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