3. Physiology
        3.13. Respiratory
            3.13.3. Gas carriage
                3.13.3.4. Oxygen
3.13.3.4.2. Oxygen dissociation curve

Oxygen dissociation curve

Important points (PO2 to SatO2)

(Assuming temp = 37C, pH = 7.40, Base excess = 0)

P50

The PO2 at which oxygen-carrying proteins is 50% saturated
--> An index of oxygen affinity
--> The lower p50 value, the high O2 affinity

For adult haemoglobin (HbA)
* p50 is at 26.6mmHg

For foetal haemoglobin (HbF)
* p50 is at 18-20mmHg
* Due to lack of 2,3-DPG binding (no beta chain)

NB:

Significance of the shape of ODC

Sigmoid shaped

Two portions:

  1. Flat upper portion
  2. Steep lower portion

Flat upper portion

Fall in alveolar pO2

--> Little change in O2 sats

Steep lower portion

As peripheral tissues extract O2

--> Drop in PO2 is not too great

--> Maintains O2 diffusion gradient

Positive cooperativity

Positive cooperativity

When one of the 4 globin chains in Hb binds with O2
--> Structural changes in Hb
--> O2 affinity of the haem of the remaining chains increased
--> "Positive cooperativity
--> Sigmoid shaped ODC

Effects of carbon monoxide

Two effects

  1. ODC is shifted to left
    * Binding of CO causes conformational change in Hb
    * O2 affinity by other subunits is INCREASED
  2. O2 content is reduced

Carboxyhaemoglobin dissociation curve is extremely left-shifted and is a rectangular hyperbola.

Myoglobin



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