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Hemoglobin-Oxygen Dissociation Curve

Jul 6, 2025

Overview

This lecture explains the hemoglobin-oxygen dissociation curve, focusing on its graphical representation and physiological significance, including the effects of different conditions on oxygen unloading from hemoglobin.

Structure of the Hemoglobin-Oxygen Dissociation Curve

  • Y-axis shows percent saturation of hemoglobin (Hb) with oxygen, indicating how much O₂ is bound to Hb.
  • X-axis shows partial pressure of oxygen (pO₂) in mm Hg.
  • The curve is sigmoidal (S-shaped), reflecting cooperative binding of O₂ to Hb.
  • At high pO₂ (lungs ≈104 mm Hg), Hb is ~98% saturated.
  • At resting tissue pO₂ (≈40 mm Hg), Hb is ~75% saturated.

Normal Oxygen Loading and Unloading

  • In the lungs, oxygen diffuses into blood until it reaches ~104 mm Hg, achieving ~98% Hb saturation.
  • In resting tissues (40 mm Hg), Hb saturation drops to ~75%, indicating oxygen unloading.
  • The difference (98% - 75% = 23%) represents O₂ unloaded to tissues at rest.
  • During exercise (pO₂ lowers to ≈20 mm Hg), Hb saturation can fall to ~30%, increasing O₂ unloading to ~68%.

Effects of Curve Shifts (Bohr and Haldane Effects)

Right Shift (Bohr Effect)

  • Curve shifts right with increased CO₂, H⁺, 2,3-BPG, and temperature.
  • More O₂ is unloaded to tissues at the same pO₂ (e.g., 38% unloaded at rest with right shift vs 23% normally).
  • Represents decreased Hb affinity for O₂.

Left Shift (Haldane Effect)

  • Curve shifts left with decreased CO₂, H⁺, 2,3-BPG, and lower temperature.
  • Less O₂ is unloaded at the same pO₂ (e.g., 13% unloaded at rest with left shift).
  • Represents increased Hb affinity for O₂.

Key Terms & Definitions

  • Hemoglobin (Hb) — protein in red blood cells that binds oxygen.
  • Percent Saturation — percentage of Hb molecules with O₂ bound.
  • Partial Pressure of Oxygen (pO₂) — measure of O₂ concentration in blood (mm Hg).
  • Bohr Effect — rightward shift in the dissociation curve due to increased CO₂, H⁺, 2,3-BPG, or temperature, enhancing O₂ unloading.
  • Haldane Effect — leftward shift in the curve due to decreased CO₂, H⁺, 2,3-BPG, or temperature, reducing O₂ unloading.
  • 2,3-BPG — molecule that decreases Hb affinity for O₂, promoting O₂ release.

Action Items / Next Steps

  • Review and understand the hemoglobin-oxygen dissociation curve and its physiological significance.
  • Study how the Bohr and Haldane effects alter oxygen delivery to tissues.
  • Know conditions that shift the curve right or left and their impact on oxygen unloading.

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