Oxygen-haemoglobin dissociation curve desaturation at low PO2

Steps:

• Identify saturation on the curve: at PO2 1.5 kPa, haemoglobin is ~25% saturated.
• At PO2 1.0 kPa, saturation falls to ~5%.
• Calculate change across the range: 5% - 25% = -20%.
• This reflects oxygen unloading as PO2 drops in tissues.

Why B is correct:

• The dissociation curve shows a 20% decrease in saturation for a PO2 drop from 1.5 kPa to 1.0 kPa, per standard physiological values.

Why the others are wrong:

• A: -6% underestimates the desaturation in this low PO2 range.
• C: 20% ignores the directional decrease in saturation.
• D: 46% overestimates, as larger changes occur in the mid-curve steep portion (2-4 kPa).

Final answer: B