CHARACTERISTICS OF RED BLOOD CELLS
Red blood cells, or erthrocytes, are tiny, biconcave disks, which are thin near their centers and the thicker around their rims. This special shape is related to the red cell's function of transporting gases in that the shape provides an increased surface area through which gases can diffuse. The shape also places the cell membrane closer to various interior parts, where oxygen-carrying hemoglobin is found.

Each red blood cell is about one-third hemoglobin by volume, and this substance is responsible for the color of the blood. Specifically, when hemoglobin is combined oxygen, the resulting oxyhemoglobin is bright red, and when oxygen is released, the resulting deoxyhemoglobin is darker.

Although the red blood cells have nuclei during their early stages of development, these nuclei are lost as the cells mature. This characteristic, like the shape of a red blood cell, seems to be related to the function of transporting oxygen, since the space previously occupied by the nucleus is available to hold hemoglobin.

RED BLOOD CELLS COUNTS
The number of red blood cells in a cubic millimeter (mm3) of blood is called the red blood cell count (RBC count). Although this number varies from time to time, the normal range for adult males is 4,600,000-6,200,000 cells per mm3, and that for adult females is 4,200,000-5,400,000 cells per mm3.

Since the number of circulating red blood cells is closely related to the blood's oxygen-carrying capacity, any changes in this number may be significant. For this reason, red blood cell counts are routinely made to help diagnose and evaluate the courses of various diseases.

DESTRUCTION OF RED BLOOD CELLS
Red blood cells are quite elastic and flexible, and they readily change shape as they pass through small blood vessels. With time, however, these cells become more fragile, and they are often damaged or ruptured when they squeeze through capillaries, particularly those in active muscles.

Damaged red cells are phagocytized and destroyed, primarily in the liver and spleen, by macrophages.

The hemoglobin molecules from the red cells undergoing destruction are broken down into molecular subunits of heme, an iron-containing portion, and globin, a protein. The heme is further decomposed into iron and a greenish pigment called biliverdin. The iron, combined with protein, may be carried by the blood to the blood-cell-forming tissue (hematopoietic tissue) in red bone marrow and reused in the synthesis of new hemoglobin. Otherwise, the iron may be stored in the liver cells in the form of an iron-protein complex. In time, the biliverdin is converted to an orange pigment called bilirubin. Biliverdin and bilirubin are excreted in the bile as bile pigments.

RED BLOOD CELL PRODUCTION
Red blood cell formation (hematopoiesis) occurs initially in the yolk sac, liver, and spleen, but after an infant is born, these cells are produced almost exclusively by the tissue that lines the spaces within the red bone marrow.

The average life span of a red blood cell is about 120 days. Although a large number of these cells are removed from circulation each day, the number of cells in the circulating blood remains relatively stable. This observation suggests that a homeostatic mechanism regulates the rate of red cell production.

The mechanism involves negative feedback, which employs a hormone called erythropoietin. In response to prolonged oxygen deficiency, erthropoietin is released, primarily from the kidneys and to a lesser extent from the liver. At high altitudes, for example where the Po is reduced, the amount of oxygen delivered to the tissues decreases. This drop in oxygen concentration triggers the release of erythropoietin, which travels via the blood to the red bone marrow and stimulates increased red cell production.

After a few days, a large number of newly formed red cells begin to appear in the circulating blood, and the increased rate of production continues as long as the kidney and liver tissues experience an oxygen deficiency. Eventually, the number of red cells in circulation may be sufficient to supply these tissues with their oxygen needs. When this happens or when the pO2 returns to normal, the release of erythropoietin ceases, and the rate of red cell production is reduced.

Figure 14.4 illustrates the stages in the development and differentiation of red blood cells, white blood cells and platelets that originate from a single type of cell (stem cell) called a hemocytoblast.

DIETARY FACTORS AFFECTING RED BLOOD CELL PRODUCTION
Red blood cell production is significantly influenced by the availability of two of the B-complex vitamins - vitamin B12 and folic acid. These substances are required for the synthesis of DNA molecules, and thus, are needed by all cells for growth and reproduction. Since cellular reproduction occurs at a particularly high rate in hematopoietic tissue, this tissue is especially affected by a lack of either vitamin.

In addition, iron is needed for hemoglobin synthesis and normal red blood cell production. Iron can be absorbed from food passing through the small intestine; also, much of the iron released during the decomposition of hemoglobin from damaged red blood cells is available for reuse. Because of iron reuse, relatively small quantities of iron are needed in the daily diet.