In anaemia, the haemoglobin (Hb) content of blood is reduced, either because there are abnormally low numbers of circulating red blood cells, or less Hb per cell. How does this change manifest itself?

Consider the relationship between the partial pressure of O2 in blood (PaO2), saturation of haemoglobin (Hb) and O2 content (figure below). The PaO2 is the partial pressure of O2 dissolved in plasma, and doesn’t consider the majority of O2 that is bound to Hb.

anaemia oxygen content dissociation

If someone has anaemia then blood leaving the lungs will have a normal PaO2 as plasma will equilibrate with normal alveolar gases. O2 saturation is a measure of what percentage (this is the important bit) of Hb is bound to O2; this will be unchanged in anaemia also, since having reduced Hb won’t prevent whatever Hb is left from becoming fully saturated. Finally, we have O2 content, which is how much O2 is actually in a given litre of blood – this is the actual amount (in ml) of O2 in blood (most of it bound to Hb). This will be reduced in anaemia, because although Hb is saturated, there isn’t as much of it in each volume of blood.

An analysis of arterial blood gas arterial blood gases (ABG) will tell you what the PaO2 is (from which you can estimate what O2 content should be), but it will fool you in anaemia by appearing normal. Haemoglobin saturation by pulse oximetry will also look normal. The key will be the Hb levels (usually 12-14 g/100 ml), which will be lower than normal.

Finally, mixed venous blood returning to the heart will have a reduced PvO2 and O2 saturation since the hungry tissues of the body will deplete blood of O2 to a greater extent, there being less O2 content to begin with in arterial blood.

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