Oxygen partial pressure versus oxygen content in anaemia
The arterial partial pressure of oxygen (the pressure that dissolved oxygen exerts) can be normal in anaemia while the oxygen content, the total carried, is low.
Remember
A normal arterial partial pressure of oxygen tells you only about the dissolved oxygen, so in anaemia the oxygen bound to haemoglobin, and therefore the content, can still be low.
Memory aid
pressure is the push of what is dissolved, content is everything in the bucket
Partial pressure is the push of the dissolved oxygen, while content counts all the oxygen carried, dissolved and bound.
Oxygen in blood is dissolved or bound to haemoglobin. Only a small proportion is dissolved, and the much larger part is bound to haemoglobin (the red-cell protein that carries most of the oxygen). The oxygen content is the total amount carried in each litre of blood, so it counts the dissolved oxygen plus the bound oxygen.
The partial pressure is a different measurement. Dissolved oxygen molecules collide with the walls of their container, and the pressure they generate within the liquid is the partial pressure of oxygen in that liquid. Partial pressure is in kPa and content is an amount per litre, so the two cannot be added directly. Oxygen bound to haemoglobin does not exert a pressure, so binding raises the content without changing the partial pressure.
In the lungs, oxygen moves from the alveoli into the blood until the two are in equilibrium (a balanced state with no net movement). The partial pressure of oxygen in blood leaving the lungs then approaches the value in the alveoli, about 13.3 kPa, and haemoglobin does not change that equilibrium pressure.
Anaemia means too little haemoglobin. There are fewer binding sites (places to hold oxygen), so the content falls. If the lungs are normal, the partial pressure in arterial blood can stay normal. The tissues may therefore be short of oxygen despite a reassuring partial pressure.
Key facts
- Content
- the dissolved oxygen plus the oxygen bound to haemoglobin, the total amount carried per litre of blood.
- Partial pressure
- set by the alveolar oxygen pressure, with which the dissolved oxygen comes into equilibrium.
- Dissolved part
- about 1.5% of the oxygen is dissolved, too little to make up for a fall in haemoglobin.
- Haemoglobin
- raises the oxygen content of blood but does not change its partial pressure.
- Anaemia
- can lower content while leaving the arterial partial pressure normal if the lungs are normal.
- Driving force
- oxygen moves because of a partial pressure difference, not a difference in the number of molecules.
Exam traps
- In anaemia the content falls, but the partial pressure does not fall with it when the lungs are normal, because the partial pressure reflects only the dissolved oxygen.
- Extra haemoglobin does not raise the partial pressure, because haemoglobin changes the total amount of oxygen carried and not the pressure set by the dissolved oxygen in equilibrium with the alveolar air.
- Most of the oxygen is carried by haemoglobin and not dissolved in plasma, because the dissolved part is small and cannot make up for a fall in haemoglobin.
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