USMLE

Hemoglobin

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Cardiovascular
  1. Cardiovascular Overview
  2. CO2 Transport
  3. Heart
  4. Cardiac Impulse
  5. Blood Pressure
  6. Arteries and Arterioles
  7. Veins and Venules
  8. Capillaries
  9. Circulation
  10. Blood
  11. Hemoglobin
  12. Blood Clotting
  • Topic Anchor: Hemoglobin (Hgb)
    • Found within red blood cells (RBCs)
    • Function
      • Transports oxygen to tissues
      • Returns carbon dioxide to lungs 
    • Structure (whiteboard)
      • Four polypeptide subunits, each with heme containing iron
        • each subunit can bind to one O2 or CO2 molecule
      • 2 states:
        • T state (“tense” state, low O2 affinity)
        • R state (“relaxed” state, high O2 affinity)
    • Exhibits positive-binding cooperativity
      • Affinity of hemoglobin for O2 increases with each O2 already bound
        • Bound O2 stabilizes the R state, and induces nearby subunits to change from T to R state
      • Sigmoidal shape (S shape) in ODC due to cooperativity effect
        • Oxygen dissociation chart (ODC) displays saturation of hemoglobin with O2 as a function of O2 partial pressure 
    • Decreased O2 affinity = increased O2 delivery/unloading to tissue
      • Cause a right-shift in ODC
      • Causes of decreased O2 affinity
        • Increased CO2
          • CO2 competes with O2 for same binding spots on Hgb
        • Decreased pH
          • protons compete for O2 (called Bohr shift)
        • Increased temperature
        • 2,3-BPG
          • Chemical in RBCs that binds to deoxygenated Hb and decreases its affinity for O2
          • In response to low-oxygen environments (high altitudes) to ensure tissues receive sufficient oxygen
          • Formed when BPG mutase converts 1,3-BPG to 2,3-BPG