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As stated previously, the erythrocytes or red blood cells are the most abundant cells in the plasma. They are biconcave disks that average about 7 μm in diameter and about 2 μm at their thickest part. They have a high surface area to volume ratio, which aids in the diffusion of oxygen and carbon dioxide into and out of the cells. Erythrocytes are deformable, which allows them to change shape as they pass through the small capillaries. The average life span of erythrocytes in the body is 120 days and they are removed by the spleen and liver. Banked blood can be kept for only about 30 to 45 days depending on the preservative. Hemoglobin, a conjugated protein with a molecular weight of about 64,500 daltons, is the major component of the erythrocytes. In addition to transporting oxygen, hemoglobin plays an important role in carbon dioxide transport and hydrogen-ion buffering. Although erythrocytes lack a nucleus and many organelles, they are able to metabolize nutrients such as glucose by glycolysis. In adults, erythrocytes, as well as leukocytes and platelets, are produced by the bone marrow whereas in the fetus, they are produced by the liver and spleen. The lower skull, vertebrae, shoulder and pelvic girdles, ribs, and sternum and epiphysis of the humerus and femur seem to be the main sites of erythrocyte synthesis in the adult. The bone marrow is a large and very metabolically active tissue. Approximately 25% of the marrow is dedicated to erythrocyte synthesis. The remainder produces leukocytes. The difference in the percent dedicated to red blood cell synthesis versus white blood cell synthesis probably reflects the difference in life span of the cell types with the white blood cells having a much shorter life span.

Hemoglobin is the main protein in the erythrocytes and is made up of four subunits each consisting of a globular-shaped protein and an iron atom. Each iron atom can bind one oxygen molecule. The hematocrit, or percent of the blood that consists of red blood cells, is generally around 40% to 45%—usually higher in men than in women. The hemoglobin concentration in the blood can be estimated as one-third of the hematocrit. A hematocrit of 45% is approximately 15 g of hemoglobin per 100 ml of blood. In a normal adult there are approximately 800 to 900 g of hemoglobin; thus, it is one of the most abundant proteins in the body.

All blood cells arise from the pluripotent hematopoietic stem cells (HSC) in the bone marrow through the process of hematopoiesis (Figure 11-1). Through a combination of specific cytokines and growth factors, and changes in the milieu of the niche in which the stem cells reside, the pluripotent cells mature through several different stages to their appropriate blood cell type. The maturation to terminal differentiation to become an erythrocyte, progresses through several morphologic stages, each having characteristic ultrastructural features. The presence of abnormal ...

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