Be honest — would you have guessed that red blood cells are
mysterious? No, I wouldn’t have either. They’re the simplest
cells in our bodies, for goodness sake — they don’t even have
DNA. All they do is carry hemoglobin around, picking up oxygen
as they pass the lungs and gradually dumping it everywhere else.
How hard can that be to understand? And we’ve studied them in
various ways for over 450 years. But indeed it turns out that
there are significant holes in our knowledge of how the number,
size and hemoglobin concentrations of red blood cells are
controlled, and how these control systems go wrong in anemia. We
do know where new red blood cells come from — the bone marrow —
and we know some of the factors that control the development and
release of new red blood cells, such as erythropoietin. The
feedback control between “too few red cells” and “more
erythropoietin needed” goes mainly through the kidneys; the
mechanism the kidney uses to sense oxygen levels (protein
hydroxylation) and induce erythropoietin