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B3.1.13 (HL)—Oxygen dissociation curves

Oxygen dissociation curves show how adult haemoglobin, foetal haemoglobin and myoglobin differ in percentage saturation, oxygen affinity and oxygen release.

Syllabus
First assessment 2025
Objective
B3.1.13
Level
HL

Exam analysis

Chance of appearing4%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper3
Typical marks1–6

Common command terms

  • Explain
  • Discuss
  • State
  • Identify
  • Describe

Scoring notes

Common mistake
Reversing graph axes or failing to label partial pressure of oxygen and percentage saturation correctly.

Recent exam appearances

May 2025Paper1B ["HL"] · TZ21(b)[ 2 ]B3.1.13 (HL)—Oxygen dissociation curves
May 2025Paper1B ["HL"] · TZ21(a)[ 1 ]B3.1.13 (HL)—Oxygen dissociation curves
May 2023Paper3 ["HL"] · TZ123(c)[ 1 ]B3.1.13 (HL)—Oxygen dissociation curves
May 2019Paper3 ["HL"] · TZ223[ 6 ]B3.1.13 (HL)—Oxygen dissociation curves
May 2017Paper3 ["HL"] · TZ121(b)[ 2 ]B3.1.13 (HL)—Oxygen dissociation curves
Practice this objective

Coverage 2012–2025 · Updated 15 Jul 2026

Oxygen Dissociation Curves Show Affinity and Loading

HL only

An oxygen dissociation curve plots haemoglobin saturation against oxygen partial pressure and shows how readily oxygen binds or is released.

The sigmoidal shape reflects cooperative binding: binding one oxygen changes haemoglobin shape and makes later binding easier. Curve position reveals affinity; a right shift means lower affinity.

Read the graph by checking: partial pressure; saturation; steep unloading region; curve shift.

A right-shifted curve reaches lower saturation at a given oxygen pressure, so it can unload more oxygen in active tissue.

A curve does not show oxygen flow or blood volume by itself; it shows binding relationship under specified conditions.

Oxygen dissociation curves

HL only

Assessment in practice

1–6 marks
How it is assessed

This objective is assessed through essay response, commonly using Explain / Discuss / State.

Command terms

Explain / Discuss / State / Identify / Describe

What earns marks

Build the answer around this relationship: Adult haemoglobin shows a sigmoid oxygen dissociation curve because of cooperative binding.

Watch for

Reversing graph axes or failing to label partial pressure of oxygen and percentage saturation correctly.

Representative question

Question 1

[Maximum number: 6]

Discuss the significance of the oxygen dissociation curves for adult hemoglobin and fetal hemoglobin.

Interpret Haemoglobin And Bohr Shift

HL only

Haemoglobin increases oxygen transport because oxygen is poorly soluble in plasma. Reversible and cooperative binding allow loading at high pO2 and unloading at low pO2. High carbon dioxide lowers pH and causes the Bohr shift, reducing affinity and promoting oxygen release in active tissues. Dissociation curves show these changes through sigmoid shape and left/right shifts.

  • Haemoglobin has four subunits with haem groups for reversible oxygen binding.
  • Bohr shift chain: more CO2 -> lower pH -> lower affinity -> right shift -> more unloading.
  • Curve interpretation needs axes, sigmoid shape, saturation, and affinity direction.

Concept essentials

  • Adult haemoglobin shows a sigmoid oxygen dissociation curve because of cooperative binding.
  • Higher oxygen partial pressure increases haemoglobin saturation until the curve plateaus.
  • Foetal haemoglobin is left-shifted because it has greater oxygen affinity than adult haemoglobin.
  • Myoglobin has very high oxygen affinity and a curve further left than foetal haemoglobin.