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Peripheral Blood

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The peripheral blood is composed of cells suspended in plasma (Table 24-1).

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Table 24–1. Normal Values for Peripheral Blood Elements. 1
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Erythrocytes (Red Blood Cells)

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Normal erythrocytes are nonnucleated biconcave disks with a uniform diameter of 7.5–8 μm. Erythrocytes are extremely pliable cells, able to change shape as they squeeze through the microcirculation. The erythrocyte cytoplasm contains hemoglobin, a protein complexed to an iron containing porphyrin that gives the cell its characteristic red color. Hemoglobin is vital to oxygen transport in the blood—the main function of the erythrocyte.

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Erythrocytes have a life span in the peripheral blood of approximately 120 days. They are derived from normoblasts in the bone marrow, which lose their nucleus in the final stage of development prior to release into the peripheral blood (Figure 24-1; see also Figure 24-3).

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Figure 24–1.
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Normal hematopoiesis. (1) Pluripotent hematopoietic stem cells or colony-forming units (CFU-S) typically are CD34-positive. Morphologically, they appear as blast cells and include the hematocytoblasts of earlier texts. (2) These partially differentiated stem cells serve as progenitor cells of one of the major series (CFU-E, erythroid; CFU-M, monocytic; CFU-G, granulocytic; CFU-Meg, megakaryocytic). They are otherwise known as erythroblasts, myeloblasts, and megakaryoblasts. (3) Morphologically, neutrophils, basophils, and eosinophils can be distinguished at the myelocyte stage. Functionally, it is believed that they develop from separate progenitor cells. [CFU-G, neutrophils; colony-forming unit, eosinophils (CFU-EO), eosinophils; colony-forming unit, basophils and mast cells (CFU-Mast), basophils and mast cells.]

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Newly released erythrocytes are slightly larger than mature cells and slightly basophilic because of the presence of residual ribosomes and mitochondria. (They demonstrate a finely reticular pattern with supravital stains such as cresyl violet—hence the term reticulocytes.) At any given time, approximately 1% of erythrocytes in the peripheral blood are newly released (eg, reticulocytes). Nucleated normoblasts are not normally found in the peripheral blood.

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Granulocytes

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There are approximately 1000 red cells ...

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