Oxygen dissociation curve

Important points (PO2 to SatO2)

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

• 0mmHg = 0%
• 10mmHg = 10%
• 26.6mmHg = 50%
  * p50 point
• 40mmHg = 75%
  * Mixed venous blood
• 60mmHg = 91%
  * "ICU" point
• 100mmHg = 97.5%
  * Arterial blood
• 150mmHg = 98.8%

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:

• p50 is a value on X-axis, not a point on the curve.

• Higher P50 => right-shifted curve => LOWER O2 affinity

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

• 240 times the affinity to O2
• Ties up Hb and make them unavailable for O2 uptake
• Presence of COHb also interfere with unloading of O2
• COHb = carboxyhaemoglobin

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

• Contains single chain
• Very low P50 of 2.75mmHg
• ODC for myoglobin is a rectangular hyperbola