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B3.1.12 (HL)—Bohr shift

The Bohr shift links high carbon dioxide, lower blood pH and reduced haemoglobin oxygen affinity, increasing oxygen release in active tissues.

Syllabus
First assessment 2025
Objective
B3.1.12
Level
HL

Exam analysis

Chance of appearing8%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper3
Typical marks1–6

Common command terms

  • Explain
  • Draw

Scoring notes

Common mistake
Drawing or describing the Bohr shift in the wrong direction when carbon dioxide increases.

Recent exam appearances

November 2025Paper1A ["HL"] · TZ123[ 1 ]B3.1.12 (HL)—Bohr shift
May 2025Paper1A ["HL"] · TZ325[ 1 ]B3.1.12 (HL)—Bohr shift
November 2024Paper3 ["HL"] · TZ021(d)[ 3 ]B3.1.12 (HL)—Bohr shift
November 2020Paper3 ["HL"] · TZ023(a)[ 3 ]B3.1.12 (HL)—Bohr shift
May 2019Paper3 ["HL"] · TZ124(b)[ 6 ]B3.1.12 (HL)—Bohr shift
Practice this objective

Coverage 2014–2025 · Updated 15 Jul 2026

The Bohr Shift Helps Active Tissues Unload Oxygen

HL only

The Bohr shift is the reduced oxygen affinity of haemoglobin when carbon dioxide rises and pH falls, promoting oxygen release in respiring tissues.

Respiration produces carbon dioxide, which forms carbonic acid and increases H⁺. These changes stabilize the lower-affinity form of haemoglobin, shifting the dissociation curve right.

Trace: more respiration; CO₂ and H⁺ rise; affinity falls; unloading increases.

During exercise, muscle CO₂ production rises, so haemoglobin releases more oxygen at the same tissue partial pressure.

The Bohr shift changes affinity, not the amount of haemoglobin or the oxygen concentration in the air.

Bohr shift

HL only

Assessment in practice

1–6 marks
How it is assessed

This objective is assessed through structured response, commonly using Explain / Draw.

Command terms

Explain / Draw

What earns marks

Build the answer around this relationship: Respiring tissues produce carbon dioxide, which lowers blood pH.

Watch for

Drawing or describing the Bohr shift in the wrong direction when carbon dioxide increases.

Representative question

Question 1

[Maximum number: 6]

Explain, with the aid of an annotated diagram, how physical exercise affects the affinity of hemoglobin for oxygen.

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

  • Respiring tissues produce carbon dioxide, which lowers blood pH.
  • Lower pH reduces haemoglobin affinity for oxygen.
  • The Bohr shift moves the oxygen dissociation curve to the right.
  • At a given oxygen partial pressure, more oxygen is released to active tissues.
  • The Bohr effect helps match oxygen delivery to tissue respiration rate.