Physiology for MRCEM Primary

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Respiratory physiology

Gas transport and Hemoglobin

[box type=”download”] –Effects of low Hb upon arterial PO2 and O2 saturation and the dissociation curve — Effects of HbF on the dissociation curve and the reasons behind this [/box]


This is an abnormally low [Hb]; the O2 capacity is therefore less and the O2 content at any PO2 is reduced.
Arterial PO2 and O2 saturation remain normal.
In order to deliver the same amount of O2 to the tissues, the capillary PO2 would have to fall further than normal, reducing the driving force for O2 diffusion into the tissues.
Diffusion can be inadequate for high metabolism states (exercise), although a 50% reduction in [Hb] does not usually cause symptoms at rest.

Fetal haemoglobin.

Fetal haemoglobin (HbF) binds 2,3-DPG less strongly than does adult haemoglobin (HbA), and so the dissociation curve is shifted to the left. This facilitates the transfer of O2 from maternal blood to the fetus, where the arterial PO2 is only 5 kPa.