The mature RBC (erythrocyte) is a biconcave disk with an average life span of 120 days. Because it lacks a nucleus and mitochondria, it is unable to synthesize protein, and its limited metabolism is barely enough to sustain it. Erythrocytes function primarily as containers for Hgb. As such, they transport oxygen from the lungs to all body cells and transfer carbon dioxide from the cells to the organs of excretion. The RBC is resilient and capable of extreme changes in shape. It is admirably designed to survive its many trips through the circulation.22
Old, damaged, and abnormal erythrocytes are removed mainly by the spleen and also by the liver and the red bone marrow. The iron is returned to plasma transferrin and is transported back to the erythroid marrow or stored within the liver and spleen as ferritin and hemosiderin. The bilirubin component of Hgb is carried by plasma albumin to the liver, where it is conjugated and excreted into the bile. Most of this conjugated bilirubin is ultimately excreted in the stool, although some appears in the urine or is returned to bile.
The hematologist determines the numbers, structure, color, size, and shape of erythrocytes; the types and amount of Hgb they contain; their fragility; and any abnormal components.