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This question is out of curiosity.

The life expectancy of a red blood cell (RBC) is approximately 3 months, and then RBCs are disposed of. Why does the body opt to build new red blood cells rather than using the existing ones? Do RBCs lose functionality/efficiency as they get older?

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They are recycled, the iron and other components are broken down and then absorbed. Adaptations of the RBC prevent the same RBC being used. They lack a nucleus to make them highly efficient oxygen carriers (pack as much haemoglobin as possible). Without a nucleus and other organelles they're unable to synthesise the stuff they'd need for renewal. There's a lot of wear and tear that occurs as they're constantly flowing in tiny blood vessels and getting damaged or affected by substances in the blood.

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    $\begingroup$ Just one additional factor worth mentioning: the haemoglobin-oxygen interaction is a source of damaging free radicals which add to the 'wear and tear'. $\endgroup$ – Alan Boyd Jul 9 '13 at 17:42
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RBCs do lose their functionality as they get older. In addition to the free radical damage they receive, the cells also become smaller over time which increases the effective concentration of the haemoglobin which causes other problems like hemolysis or rigidity.

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    $\begingroup$ I didn't know this - do you have a reference? $\endgroup$ – Alan Boyd Jul 12 '13 at 11:47

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