Physiology for MRCEM Primary

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Respiratory physiology

Oxygen transport


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— Haemoglobin as a co-operative binder of oxygen molecules
— Relation between blood oxygen content and PO2 (oxygen dissociation curve)
— Factors shifting the dissociation curve to the left or right (pH, PCO2, temperature) [/box]

The O2 solubility in plasma is low and at a PO2 of 13 kPa blood contains only 3 mL/L of dissolved O2 in solution.
Most O2 is therefore carried bound to haemoglobin.
Each gram of haemoglobin can combine with 1.34 mL of O2.
The actual amount of O2 bound to haemoglobin (O2 content) depends on the PO2, and the percentage O2 saturation = content/capacity × 100.
Each haemoglobin molecule binds up to four O2 molecules.
Binding is cooperative, so that the binding of each O2 molecule makes it easier for the next.
This steepens the O2 haemoglobin dissociation curve.
The curve flattens above ∼8 kPa PO2 as all binding sites become occupied.
Thus, for a normal arterial PO2 (∼13 kPa) and [Hb], the blood is ∼97% saturated and contains slightly less than 200 mL/L of O2.
Because the dissociation curve is flat in this region, any increase in PO2 (breathing O2-enriched air) will have little effect on content.
On the steep part of the curve, however (<8 kPa PO2), small changes in POwill have large effects on content.

Oxygen uptake and delivery.

The high PO2 in the lungs facilitates O2 binding to haemoglobin, whereas the low PO2 in the tissues encourages release.
The dissociation curve is shifted to the right (reduced affinity, facilitating O2 release) by a fall in pH, a rise in PCO2 (Bohr shift) and an increase in temperature, which occur in active tissues. The metabolic by-product 2,3-diphosphoglycerate (2,3-DPG) also causes a right shift.
In the lungs, PCO2 falls, the pH consequently rises and the temperature is reduced; these all increase affinity and shift the curve to the left, facilitating O2 uptake.

Carbon monoxide.

Carbon monoxide (CO) binds 240 times more strongly than Oto haemoglobin and, by occupying O2-binding sites, reduces the O2 capacity.
However, unlike anaemia, CO also increases the affinity and shifts the dissociation curve to the left, making O2 release to the tissues more difficult. Thus, if 50% of haemoglobin is bound to CO, PO2 needs to fall much further than in anaemia to release the same amount of O2, causing symptoms of severe hypoxia (headache, convulsions, coma, death).