Oxygen content at altitude and pulmonary oedema
A mountaineering team ascends from sea level to 5,500 m, where atmospheric pressure is 50 kPa and inspired oxygen partial pressure is about 10.0 kPa. At sea level, a climber has haemoglobin concentration 150 g dm−3, arterial oxygen saturation 98%, and arterial PO2 13.3 kPa. After 10 days at altitude, haemoglobin is 180 g dm−3, saturation is 85%, and arterial PCO2 is lower than at sea level. Oxygen capacity is 1.34 cm3 O2 per g haemoglobin; dissolved oxygen is 0.003 cm3 O2 per 100 cm3 blood per kPa PO2. The climber's ventilation rate rises from 12 to 28 breaths min−1 with no change in tidal volume. A teammate develops high-altitude pulmonary oedema, with fluid in the alveoli. The team doctor claims the increased haemoglobin is the most important acclimatisation, while a guide argues that increased ventilation matters more.
1. Calculate the climber's arterial oxygen content at sea level and after acclimatisation. Show your working and include units.
Read the worked answer
Use oxygen content = (Hb × 1.34 × saturation) + dissolved O2, with Hb in g dm−3 and capacity in cm3 per dm3. Since 1 dm3 = 10 dL, divide by 10 for cm3 per dL.
Sea level
- Hb = 150 g dm−3 = 15 g dL−1.
- Capacity = 15 × 1.34 = 20.1 cm3 dL−1.
- Bound O2 = 20.1 × 0.98 = 19.7 cm3 dL−1.
- Dissolved O2 = 0.003 × 13.3 = 0.040 cm3 dL−1.
- Total ≈ 19.7 cm3 dL−1.
After acclimatisation at altitude
- Hb = 180 g dm−3 = 18 g dL−1.
- Capacity = 18 × 1.34 = 24.1 cm3 dL−1.
- Bound O2 = 24.1 × 0.85 = 20.5 cm3 dL−1.
- Dissolved O2 = 0.003 × 10.0 = 0.030 cm3 dL−1.
- Total ≈ 20.5 cm3 dL−1.
So arterial oxygen content is maintained, even slightly increased, despite the fall in saturation from 98% to 85%. Without the rise in haemoglobin, content would be 15 × 1.34 × 0.85 + 0.03 ≈ 17.1 cm3 dL−1, about 3.4 cm3 dL−1 lower.
2. Evaluate the doctor's and the guide's claims. Which acclimatisation matters more for maintaining oxygen supply? Use the calculated values.
Read the worked answer
The doctor is broadly right about arterial oxygen content: the haemoglobin increase from 15 to 18 g dL−1 raises capacity by 20%, adding about 3.4 cm3 dL−1 compared with no acclimatisation. That is the main reason content is maintained. But the guide is right that ventilation is essential, because it raises alveolar PO2 and therefore saturation; without increased ventilation, saturation would fall further. Increased ventilation also lowers PCO2, causing respiratory alkalosis. This shifts the oxygen dissociation curve left, which helps loading in the lungs but makes unloading in tissues harder. Over days, increased 2,3-BPG shifts the curve back to the right, improving unloading.
The claim that one is 'most important' depends on the measure. For maintaining arterial oxygen content, the haemoglobin increase is quantitatively large. For protecting saturation and acid-base balance, ventilation is indispensable. They are complementary, not alternatives.
3. Explain how pulmonary oedema impairs gas exchange and why supplemental oxygen may help only partly.
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Pulmonary oedema puts fluid in the alveoli. This increases the diffusion distance between alveolar air and blood, reduces the surface area available for gas exchange, and can cause ventilation-perfusion mismatch as some alveoli are poorly ventilated but still perfused. Oxygen is much less soluble in water than carbon dioxide, so oxygen diffusion is impaired earlier and more severely; the climber becomes hypoxaemic before CO2 retention is severe.
Supplemental oxygen raises alveolar PO2 and steepens the diffusion gradient, so it can improve saturation. However, if alveoli are fluid-filled or collapsed, oxygen cannot reach the blood in those regions, and blood passing through them remains desaturated, a shunt. Oxygen therapy therefore helps but may not fully correct hypoxaemia. The underlying oedema must be treated, and the climber should descend.