Steep portion of oxygen dissociation curve enables efficient unloading

Steps:

• Recall sigmoidal oxygen-haemoglobin dissociation curve: flat at high PO2 (lungs), steep at tissue PO2 (2-6 kPa), flat at low PO2.
• Note resting muscle PO2 ≈5.3-7 kPa (high saturation, ~90-95%), active muscle PO2 drops to ≈2.7-4 kPa (saturation falls to ~50-70%).
• In steep region (4-7 kPa), small PO2 decrease causes large saturation drop, unloading ~2 O2 per haemoglobin (Hb carries 4 total).
• This explains greater oxygen delivery to active vs. resting muscle.

Why B is correct:

• Between 4-7 kPa lies the curve's steep slope, where small ΔPO2 unloads ~2 O2/Hb per Bohr effect definition, maximizing tissue delivery.

Why the others are wrong:

• A: Omits critical 4-7 kPa range; small ΔPO2 unloads little outside steep region.
• C: Includes sigmoidal shape but ignores specific pressure range needed for ~2 O2 unloading.
• D: Incorrectly states 4 O2 unload (full saturation drop requires larger ΔPO2); range unspecified.

Final answer: B