The Relationship Between Arterial pO2 and Oxygen Saturations

The majority of oxygen in transported blood is bound to haemoglobin (Hb). This is measured as oxygen saturation (SaO2). Only 3% is found in a dissolved state, the partial pressure of which is measured in kilopascals (kPa). Normal adult Hb consists of two α and two β polypeptide chains, each attached to an O2-binding iron-containing haem molecule.

As O2 binds to each haem molecule, the haem changes shape (allosteric change) influencing the further binding of O2 to the other haem molecules in the haemoglobin (i.e. binding occurs more readily).

The amount of O2 bound to Hb is related to the partial pressure of O2 in the blood. This is shown in the oxy-haemoglobin dissociation curve, a sigmoid shaped curve.

The curve can shift to the right or the left, depending on factors such as blood pH, temperature, etc. When there is right shift of the curve, the p50 is at a higher partial pressure. Therefore higher O2 levels are needed to saturate 50% of the Hb (i.e. there is lower affinity of Hb for O2). This enhances O2 delivery at the tissues. When there is left shift of the curve, the p50 is at a lower partial pressure. Therefore, Hb holds onto O2 more at a tissue level but will extract it better at the pulmonary capillaries.

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