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LEARNING OBJECTIVES

Learning Objectives

  • The student will be able to define and derive the six standard erythrocyte indices as evaluated in a complete blood count.

  • The student will be able to calculate dissolved and bound O2 contents of blood when given data for its Po2, percent oxygenation, and hemoglobin concentration ([Hb]).

  • The student will be able to identify basic features of an HbO2 dissociation curve including its P50, and the effects of pH, Pco2, [2,3-DPG], temperature, carbon monoxide, and common genetic changes on the P50.

  • The student will be able to describe the major mechanisms by which erythrocytes promote CO2 transport, and estimate the compartment sizes of CO2 carried in dissolved, Hb-bound, or dissociated states.

The major function of erythrocytes or red blood cells (RBCs) is transport of the respiratory pigment hemoglobin (Hb). In their mature form, erythrocytes are biconcave discs 7-8 μm in diameter, with thickness of 0.5-2.0 μm, and an average volume of 80-90 μm3 or fL. Whole blood contains large numbers of RBCs, typically 4.5-6.0 × 106/μL. Although living cells, mature erythrocytes are anucleate and contain few organelles. During a typical functional life span of 120 days, each RBC may travel 1,000 km from synthesis in the marrow until destruction in liver, spleen, or other site. Factors that regulate RBC production and longevity are beyond the scope of this book, except as they influence O2 and CO2 movements across alveolar septa and into blood vessels that originate in the right ventricle and traverse the lungs before returning to the left ventricle.

THE SIX STANDARD ERYTHROCYTE INDICES

As part of a complete blood count (CBC) done on each hospital admission, the following three indices are routinely measured on a patient’s RBCs:

  • Hematocrit—the fraction of whole blood that is composed of erythrocytes, most commonly expressed in percent. Historically, blood was placed in glass capillary tubes and centrifuged until the erythrocytes sedimented together at the bottom, with a "buffy coat" of leukocytes and platelets above them, and clear, amber-colored plasma on top. Normal values for hematocrit (Hct) vary with age and sex over predictable ranges (Fig. 3.1).

  • Hemoglobin Concentration—the quantity of hemoglobin (Hb) per unit of blood, usually expressed as g/dL or simply g%. Historically, hemoglobin concentration ([Hb]) was determined by diluting whole blood with potassium ferricyanide to lyse the RBCs and oxidize ferrous ions (Fe2+) to ferric ions (Fe3+), thus converting normal Hb into methemoglobin (metHb) that cannot bind O2. MetHb combines with cyanide ions to form a quantifiable color product, cyanomethemoglobin. [Hb] also varies with age and sex (Fig. 3.1).

  • Red Blood Cell Count—the density of erythrocytes in whole blood, typically reported as ×10 6/μL. Due to their great numbers and small size, ...

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