Oxygen Content

As we have seen, the total volume of oxygen carried in arterial blood at any time is the sum of that carried by the haemoglobin (98%) plus that dissolved in the plasma (2%). This total is termed the arterial oxygen content.

The oxygen content is dependent upon:

• ⦁ The haemoglobin concentration [Hb] in the blood
• ⦁ The oxygen carrying capacity of the haemoglobin OCC Hb
• ⦁ The saturation of haemoglobin with oxygen SaO2 (Hb)
• The oxygen dissolved in the plasma

The volume of oxygen dissolved in plasma from arterial blood is directly proportional to the PaO₂ and is approximately: 0.23 ml per litre blood per kPa PaO_2.

Therefore:
Arterial oxygen content = ([Hb] x OCC Hb x SaO2(Hb)) + (0.23 x PaO2)

Learning bite

Increasing the PaO₂ above 10kPa has a major effect on increasing the volume of oxygen dissolved in the plasma.
Decreasing the PaO₂ leads mainly to a reduction in the volume of oxygen carried by haemoglobin.
Even at a low PaO₂ there is still a considerable amount of oxygen in the blood which can act as an oxygen reserve.

Under normal circumstances

• The haemoglobin concentration = approximately 130-165 g/l in men, 115-150 g/l in women
• The oxygen carrying capacity of Hb = 1.34 ml/g
• The SaO₂ = 97%
• The volume of oxygen dissolved in plasma = 0.23 ml/l/kPa

It follows that the oxygen content per litre of arterial blood (i.e. the amount associated with the haemoglobin molecule as well as that dissolved in the plasma) is equal to:

[Hb] x OCC Hb x SaO₂(Hb) + 0.23 x PaO₂
Oxygen content of haemoglobin + Oxygen content of the plasma