Steepest slope on oxygen-hemoglobin dissociation curve

Steps:

• Recall the sigmoid shape of the oxygen-hemoglobin dissociation curve, where saturation rises slowly at low pO2, steeply in the middle, and plateaus at high pO2.
• Compare saturation changes across ranges: calculate approximate delta % saturation using standard curve values (0 kPa: 0%, 5 kPa: ~40%, 10 kPa: ~90%, 15 kPa: ~95%, 20 kPa: ~97%).
• Identify largest delta: 0-5 kPa (~40%), 5-10 kPa (~50%), 10-15 kPa (~5%), 15-20 kPa (~2%).
• Select range with maximum change in saturation.

Why B is correct:

• The curve's cooperative oxygen binding creates steepest slope at 5-10 kPa (physiological venous-arterial range), maximizing saturation change per kPa via allosteric hemoglobin transition.

Why the others are wrong:

• A: Curve starts flat due to low-affinity T-state hemoglobin, yielding smallest initial rise.
• C: Slope begins flattening as hemoglobin nears full saturation in R-state.
• D: Near-total saturation on plateau means minimal additional binding.

Final answer: B