Oxyhaemoglobin dissociation curve

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Oxyhaemoglobin dissociation curve

An ideal oxygen transport medium would take up oxygen rapidly and easily to become saturated in as close contact as it gets with air, and unload a proportion sufficient for normal operation of that oxygen rapidly in the tissues without requiring an unduly large difference in oxygen tensions. It would also retain a stock of oxygen for abnormal operating conditions, and unload all of that at oxygen tensions sufficiently high to sustain function, or at least cell life.

Haemoglobin comes reasonably close, in the environment of the red corpuscle. The realisation that the oxygen dissociation curve fits basic physiological needs and could be linked via Harvey's model of the circulation with the possibility of measuring cardiac output permitted the routine and exact determination of the amount, transportation and consumption of oxygen. This allowed basic cardiac and respiratory physiology to be understood.

The Hb dissociation curve is affected by changes that occur in its travels, unsurprisingly in the direction of advantage at each end of the transport chain. Examples of this:

  • Oxygen affinity is decreased by the presence of 2,3-diphosphoglycerate (DPG). Low DPG levels in the lung relative to tissue will cause human hemoglobin to have a high affinity for oxygen.
  • In the presence of carbon dioxide haemoglobin oxygen affinity decreases, helping the release of oxygen as carbon dioxide accumulates through metabolism. This is known as the Bohr effect. The CO2 diffuses into erythrocytes where carbonic anhydrase catalyzes its reaction with water (H2O to form carbonic acid, H2CO3 which is in a dissociated dynamic equilbrium with intracellular hydrogen ion, H+ and bicarbonate, HCO3-. Intracellular bicarbonate diffuses out into the blood plasma in exchange for passive diffusion of chloride, Cl-. This is the erythrocyte chloride shift.
  • Increased temperature decreases oxygen affinity
  • Alkalosis increases oxygen affinity (ie hyperventilation will reduce tissue oxygen delivery)
  • Carboxyhaemoglobin increases increases oxygen affinity (another reason why carbon monoxide is a poison)
    • Treatment high partial pressures O2
  • Methaemoglobin increases oxygen affinity (thus the toxicity of the following that can oxidise Fe2+ to Fe3+)
    • Sulphonamides
    • Aniline dyes
    • Chlorate salts
    • Nitrate salts
    • Nitrite salts
    • The former analgesic agent phenacetin
      • Treatment methylene blue which is converted to leucomethylene blue, which in turn is able to reduce methaemoglobin to haemoglobin (too much methylene blue can produce methaemoglobin).

Both myoglobin and fetal haemoglobin have higher affinities for oxygen than normal human haemoglobin.

History

  • 1857 Lothar Meyer showed that the amount of O2 released under reduced pressure by the blood was less than expected by Dalton's law
  • 1864 Felix Hope-Seyler isolated crystalline haemoglobin and described its spectrum, as well as oxyhaemoglobin when combined with oxygen
  • 1878 Paul Bert, professor of Physiology at the Sorbonne, reports his classic experiments with dog blood at various PaO2 where at body temperature blood loses half of its oxygen but the PaO2 is only reduced by a fifth.[1]
  • 1904 Bohr, Hasselbach and Krogh describe 'S' shaped dissociation curve.[2]
  • 1910 Hill's equation. SHbO2=[HbO2]/[Hb]=KHbO2[O2]n/(1+KHbO2[O2]n) where n=Hill's coefficient[3].

References

  1. Bert P. La Pression Barométrique. Recherches de Physiologie. Paris: Masson 1878
  2. Bohr C, Hasselbach KA, Krogh A. Ueber einen in biologischer beziehung wichtigen einfluss, den die kohlensäurespannung des blutes auf dessen sauerstoffbindung übt. Skand Arch Physiol 1904; 16: 402-12
  3. Leow MK. Configuration of the hemoglobin oxygen dissociation curve demystified: a basic mathematical proof for medical and biological sciences undergraduates. Advances in physiology education. 2007 Jun; 31(2):198-201.(Link to article – subscription may be required.)
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