Sigmoidal oxygen-hemoglobin dissociation curve

Steps:

• Recall the curve shows low saturation at low pO2 (0% at 0 kPa), rapid increase in mid-range, and plateau near 100% at high pO2 (95% at 14 kPa).
• Identify sigmoidal pattern: slow rise initially (0-4 kPa), steep between 4-7 kPa, then gradual to asymptote.
• Evaluate options against endpoints: must start low at 2 kPa, accelerate, and near 95% by 12-14 kPa.
• Select option with progressive, non-linear increases matching curve kinetics.

Why C is correct:

• Aligns with cooperative binding in hemoglobin, yielding ~10% at 2 kPa (slow), ~55% at 4 kPa (start of steep phase), ~80% at 7 kPa (mid-steep), and ~93% at 12 kPa (nearing plateau at 95% by 14 kPa).

Why the others are wrong:

• A: Saturations remain too low (40% max at 12 kPa), ignoring steep mid-curve rise.
• B: Gradual increases (65% at 12 kPa) contradict rapid saturation gain expected before 14 kPa.
• D: Unrealistically high early values (60% at 2 kPa) violate low-affinity initial binding.

Final answer: C