| Erythrocytes are not true cells, as they do not possess nuclei and intracellular organelles. They are cellular remnants, containing specific proteins and ions, which can be present in high concentrations. Erythrocytes are the end-product of erythropoiesis in the bone marrow, which is under the control of erythropoietin produced by the kidney (Fig. 3.1). Hemoglobin is synthesized in the erythrocyte precursor cells - erythroblasts and reticulocytes - under a tight control dictated by the concentration of heme, the synthesis of which involves the chelation of reduced ferrous iron (Fe2+) by 4 nitrogen atoms in the centre of a porphyrin ring (see Chapter 28). The main functions of erythrocytes are the transport of oxygen and the removal of carbon dioxide and hydrogen ion; as they lack cellular organelles, they are not capable of protein synthesis and repair. As a result, erythrocytes have a finite life span of 60-120 days before being trapped and broken down in the spleen.
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