oxygen dissociation curve

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    • Saturation is the percentage of Hb-binding sites occupied by O2.
    • oxygen capacity of Hb in blood is approximately 20 mL O2/100 mL of blood or 20 vol%. (MCQ)
      • Each gram of Hb has an oxygen capacity of 1.34 mL O2, (MCQ)
      • 100 mL of blood contains 15 g Hb
      • completely oxygenated blood contains approximately 20 mL O2/100 mL (1.34 mL O2 × 15 g Hb/100 mL).
    • Physiologic implications of the oxyhemoglobin dissociation curve
      • Hb combines rapidly and reversibly with O2 to form oxyhemoglobin.
      • The saturation curve has a sigmoid shape because oxygenation of the first
      • heme group of the Hb molecule increases the affinity of O2 for the other         
      •  heme groups.

The O2 capacity

    • maximum amount of O2 that can be bound to Hb
    • determined by the Hb concentration in blood.
  • The O2 content
    • total amount of O2 carried in the blood whether bound or dissolved in solution.
  • Several factors influence the oxyhemoglobin dissociation curve
    • Shifts to the right (MCQ)
      • occur when the affinity of Hb-binding sites for O2 is decreased
      • it is easier for tissues to extract oxygen.
      • Causes of this shift include  (MCQ)
        • increased CO2 (Bohr effect)
        • increased H+ (decreased pH)
        • increased temperature
        • increased 2,3-diphosphoglycerate (2,3-DPG).
      • Anemia is characterized by a reduced Hb concentration in blood and decreased arterial oxygen content.
    • Shifts to the left (MCQ)
      • occur when there is increased affinity of Hb for O2
      • it is more difficult for tissues to extract oxygen.
      • Causes of this shift include(MCQ)
        • decreased temperature
        • decreased PCO2
        • decreased H+ (increased pH),
        • decreased 2,3-DPG.
      • Stored blood loses 2,3-DPG and fetal Hb, and both decreases shift the curve to the left. (MCQ)
      • Polycythemia is characterized by a higher than normal concentration of Hb in the blood, a shift to the left in the oxyhemoglobin dissociation curve, and increased arterial oxygen content. (MCQ)
  • Carbon monoxide poisoning
    • Carbon monoxide (CO) has a much greater affinity (more than 200 times) for Hb than does O2. (MCQ)
    • Thus, the amount if CO dissolved in plasma is essentially zero.
    • The O2-binding capacity of Hb and the O2 content of blood decrease.
    • The oxy hemoglobin dissociation curve shifts to the left. (MCQ)
    • Arterial PO2 is normal,but O2 saturation of Hb decreases.

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  • Saturation versus partial pressure.
    • Each saturation curve has a single P50
    • which is the PO2 that gives 50% saturation
    • Normal P50 = 26 mm Hg (MCQ)

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  • O2 content versus partial pressure for two diffe ent hemoglobin (Hb) concentrations.
    • Curve A represents normal Hb levels in blood (15 g/100 mL).
    • Curve B represents a reduced concentration of Hb in blood (7.5 g/100 mL). The main effect of the lower Hb concentration is a reduced carrying ca- pacity of the blood.
    • Thus, in curve B, the total amount of O2/100 mL of blood is around 10 mL O2/100 mL, instead of the normal 20 mL O2/100 mL. (MCQ)

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