Week 13 Physiology

Reaction between Hb and O2.

  • O2 in blood is mainly attached to Hb but also is dissolved at a rate in proportion to PaO2. 0.003ml/mmHg (Henry’s law)


O2 – Hb dissociation curve: (This is important).Oxyhaemoglobin_dissociation_curve

  • OxyHb curve is the one that shifts. Upto 50mmHg the rise in O2 sats is fast then flattens out
  • 40mmHg – 75%, 100mmHg – 97%
  • Anaemia doesn’t affect O2 Sats but it does affect O2 concentration (amplitude of curve is less) so Hb can off load easier in tissue.
  • Right shift (think hot, acidic, hypercarbic, hypoxic low DPG muscle)
  • Even a small amount of CO causes L shift


Be able to draw it and plot in some values

  • PO2/ SaO2
    • 10 10
    • 20 35
    • 26 (P50) 50
    • 40 75
    • 50 80
    • 60 90
    • 70 92
    • 80 94
    • 90 96
    • 100 98


Be able to draw dissociation curves for Myoglobin and foetal Hb.


fetalhb_myoglobin_hemoglobin disociationfetal and normal hemoglobin







  • Definition – a weak acid or base which acts to maintain the pH at a chosen value(7.4)
  • Main buffers in vivo.
    • Bicarbonate
    • Ammonia
    • Phosphate
    • haemaglobin


CO2 transport.

  • CO2 is carried in blood as
    • Dissolved (10%)
    • Bicarbonate (60%)
      • Carbonic anhydrase in RBC speeds it up
      • Chloride shift/Hamburger effect
      • When H2CO3 formed it spontaneously disassociated into HCO3- and H+. H+ cant diffuse as easily, but HCO3 can diffuse out, to replace the anion, Cl- comes in.
    • Combined with proteins as carbamino compounds(30%)
  • CO2 dissociation curve is steeper and more linear than O2, moves depending on O2 saturation to R with increasing sats.
  • The Haldane effect is a property of haemoglobin. Deoxygenation of the blood increases its ability to carry carbon dioxide, and for Hydrogen ions. Conversely, oxygenated blood has a reduced capacity for carbon dioxide. Helps load CO2 in periphery and unload CO2 in the lung.


Viva questions:

  1. Tell me about the structure of haemoglobin.
    • 2 pairs of globin with haem molecule (not expected to draw the structure)
  2. Tell me about the reaction of Hb with O2.
  3. Draw the Hb-O2 dissociation curve.
  4. What are the advantages conferred by it’s non-linear shape ? (See West).
  5. What factors move the curve, and in which direction ?
  6. What is the definition and typical value of the P50 ? 26mmHg
  7. What is the Henderson-Hasselbalch equation ?
    • For HA + H2O –> HO3+  +  A-
    • Hasselbalch equation  
  8. What is the definition of a buffer ?
  9. What are the main buffers in the body ?
  10. Tell me about CO2 transport.
  11. Draw the CO2 dissociation curve
  12. What is the chloride shift ?
  13. What is the Bohr effect
  14. Hb/O2 affinity is inversely proportional to CO2 and acidity
  15. What is the Haldane effect