Oxygen Dissociation and Altitude
A trekker ascends from sea level to 3800 m over four days. At altitude, her arterial partial pressure of oxygen (PaO2) falls from about 95 mmHg to about 55 mmHg. Her haemoglobin concentration is unchanged at 140 g/L. Her ventilation rate increases, which lowers her arterial partial pressure of carbon dioxide and raises her blood pH slightly. At sea level her haemoglobin was about 98% saturated; at 3800 m it is about 85%. She feels breathless on exertion but is not cyanosed at rest. A companion with the same haemoglobin but a chronic lung condition has a PaO2 of 55 mmHg and is far more breathless.
1. Explain why a fall in PaO2 from 95 to 55 mmHg produces a saturation drop of only about 13 percentage points, and why this matters for her ability to exercise.
Read the worked answer
The oxygen–haemoglobin dissociation curve is sigmoidal, not linear. The steep part lies at low partial pressures; the upper part is relatively flat.
- At 95 mmHg the curve is on the plateau, so haemoglobin is near-saturated and small changes in PaO2 barely change saturation.
- At 55 mmHg the point has moved down onto the shoulder of the curve, where the slope is steeper. That is why a 40 mmHg fall in pressure produces a 13-point fall in saturation rather than a proportional 40% fall.
This plateau is protective: it means moderate falls in alveolar oxygen do not proportionally reduce oxygen carriage. Her 85% saturation at 140 g/L still carries substantial oxygen.
Why it matters for exercise: at rest, 85% saturation is adequate. On exertion, muscle oxygen demand rises, and the tissues must extract more oxygen from blood that is already less saturated. The margin between delivery and demand narrows, so she becomes breathless sooner. The curve's shape means she retains most of her carriage at rest but has less reserve to draw on during exercise - the judgement is about reserve, not resting adequacy.
2. Her rising ventilation lowers CO2 and raises pH. Predict the effect on the dissociation curve and explain whether this helps or hinders oxygen unloading at the tissues.
Read the worked answer
A rise in pH (and fall in CO2) shifts the curve left - the Bohr effect operating in reverse. Left shift means haemoglobin binds oxygen more tightly at any given partial pressure.
Effects:
- Loading at the lungs: a left shift slightly favours uptake in the pulmonary capillaries, which is helpful at altitude.
- Unloading at the tissues: tighter binding means haemoglobin releases oxygen less readily where it is needed. This is a hindrance.
There is a tension here: the same hyperventilation that raises alveolar oxygen also alkalinises the blood and impairs unloading. In practice, at the tissues local CO2 and acid production from exercising muscle shift the curve back to the right, partly offsetting this. The judgement is that hyperventilation is a net benefit for loading but carries a cost for unloading, and that cost is greatest at rest and least during active exercise where local conditions counteract it.