Red Blood Cells (RBC's)
Size: 6.7 to 7.7 μ in diameter.
Cytoplasm: Pink in color.
The mature red blood cell is a nonnucleated, round, biconcave cell.
Erythropoiesis: The main function of the red blood cell is to transport oxygen to the tissues. Production of red blood cells (erythropoiesis) is initiated by a hormone produced by the kidney called erythropoietin. When a person’s hemoglobin level is below normal, his tissues will not receive an adequate supply of oxygen, and this will stimulate the kidneys to increase their production of erythropoietin. The increased erythropoietin will then stimulate the stem cells of the bone marrow to differentiate into the pronormoblast, and there will be an increased number of red blood cells produced. As the red cells are maturing they undergo several cellular divisions. Once the orthochromic normoblast stage is reached, however, the cell is no longer capable of mitosis but will continue to mature in the bone marrow. The reticulocyte remains in the marrow for approximately two days and is then released into the peripheral blood. The red cells of the circulating blood have a lifespan of approximately 120 days, ±20 days.
Hemoglobin structure and synthesis: Hemoglobin is made up of the protein, globin, and heme. In normal adult hemoglobin, the globin portion of each molecule consists of four polypeptide chains: two α and two β chains. These chains, in turn, are composed of 141 and 146 amino acids (arranged in a specific sequence), respectively. Each chain is bent and coiled. The heme group is composed of four pyrrole rings connected by methene bridges. In the center of this structure is an atom of iron to which oxygen is attached, when the iron is in the ferrous state (Feˉˉ).
One heme molecule will be attached to each of the α and β chains. Two α and two β chains come together to form a tetramer. The single hemoglobin molecule, therefore, consists of two α chains, two β chains, and four heme groups (thus, four atom of iron). Mature red blood cells are incapable of hemoglobin synthesis. The production of heme and globin takes place independently of each other, beginning in the polychromatic normoblast, and ending in the reticulocyte stage.
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