Red blood cells & the absence of nuclei...?

Question

How is the absence of a nucleus and other organelles from (mature mammalian) erythrocytes related to their function in oxygen transport, as well as to their relatively short life span?

Answer 1

The lack of a nucleus in mammalian erythrocytes allows for them to have their distictive biconcave disk shape. This is hypothesised to permit:
[1] a larger surface area to volume ratio, optimising the exchange of oxygen
[2] flexibility, allowing them to become distorted while passing through narrow capillaries.

However, the lack of a nucleus means that erythrocytes cannot synthesise new proteins, and therefore their lifespan is neccessarily limited, as they cannot repair or replace lost or damaged proteins. Additionally, they must be replaced by differentiation from progenitor cells in the bone marrow - as, lacking a nucleus, they cannot undergo mitosis themselves.

Interestingly, in other vertebrates than mammals, the erythrocyes retain their nuclei (though they are still replaced from progenitor cells, and do not typically undergo mitosis themselves). It is not known why this is the case, and this might mean that the above speculative reasons for erythrocyte denucleation in mammals are incorrect. Birds, for example, generally have higher metabolic rates than mammals, but their erythrocytes are much larger, and retain their nuclei.

Answer 2

The absence of a nucleus and other organelles specializes the cell so it can carry more oxygen, however it also means the cell can't do anything else like reproduce. This also explains their short lifespan since they can't repair any damage done to them or really live by themselves. They are more like buckets that carry oxygen around to the body and buckets don't repair themselves, or do anything besides hold stuff; just like red blood cells.

According to Wikipedia: http://en.wikipedia.org/wiki/Red_blood_cells
"Red blood cells are also known as RBCs, haematids, or erythrocytes (from Greek erythros for "red" and kytos for "hollow", with cyte nowadays translated as "cell"). A schistocyte is a red blood cell undergoing fragmentation, or a fragmented part of a red blood cell....

The process by which red blood cells are produced is called erythropoiesis. Erythrocytes are continuously being produced in the red bone marrow of large bones, at a rate of about 2 million per second. (In the embryo, the liver is the main site of red blood cell production.) The production can be stimulated by the hormone erythropoietin (EPO), synthesised by the kidney; which is used for doping in sports. Just before and after leaving the bone marrow, they are known as reticulocytes which comprise about 1% of circulating red blood cells. Erythrocytes develop from committed stem cells through reticulocytes to mature erythrocytes in about 7 days and live a total of about 120 days. The erythrocyte undergoes changes in its plasma membrane making it susceptible to recognition and subsequent phagocytosis in the spleen, liver and bone marrow. Much of the important breakdown products are recirculated in the body. The heme constituent of hemoglobin are broken down into Fe3+ and biliverdin. The biliverdin is reduced to bilirubin, which is released into the plasma and recirculated to the liver bound to albumin. The iron is released into the plasma to be recirculated by a carrier protein called transferrin. Almost all old erythrocytes are removed from the circulation before they are old enough to hemolyze. Hemolyzed hemoglobin is bound to a protein in plasma called haptoglobin which is not excreted by the kidney."

Answer 3

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