IB Biology HLGas Exchange SystemsPaper 1 & 2~10 min read
The Bohr Shift
A hard-working muscle needs more oxygen than a resting one. Nothing measures that need and sends extra supplies – instead, the waste product the muscle makes does the job itself. Carbon dioxide loosens haemoglobin’s grip on oxygen, so the busiest tissues automatically get the most.
📚 What you need to know
Changes in the oxygen dissociation curve caused by carbon dioxide are called the Bohr effect or Bohr shift.
When the partial pressure of carbon dioxide is high, haemoglobin’s affinity for oxygen is reduced.
This happens because CO2 lowers the pH of the blood: CO2 + water → carbonic acid → hydrogencarbonate ions + hydrogen ions.
The hydrogen ions bind to haemoglobin, which changes its shape and causes it to release oxygen.
On a graph, the curve shifts to the right when CO2 levels rise.
A right shift means that at any given partial pressure of oxygen, the percentage saturation is lower.
This is useful: haemoglobin gives up its oxygen more readily in respiring tissues, exactly where it is needed.
What the Bohr shift actually is
The oxygen dissociation curve is not one fixed line. Its position depends on the conditions the blood is in, and the biggest influence is carbon dioxide.
Where cells are respiring hard, they release carbon dioxide as waste. The blood passing them therefore has a high partial pressure of carbon dioxide. In those conditions haemoglobin holds oxygen less tightly – its affinity falls – and it releases more of its load.
The chemistry, one step at a time
Read it as a loop: the harder a tissue respires, the more carbon dioxide it makes, and the more oxygen it is handed in return.
Carbon dioxide does not push oxygen off the haemoglobin. It works indirectly, by making the blood more acidic, and it is the hydrogen ions that bind to the protein and change its shape. Saying “H+ binds to haemoglobin” is what turns a description into an explanation.
What the shift looks like on a graph
Draw the curve for blood with a low carbon dioxide level, then draw it again for blood with a high level. The second curve sits to the right of the first.
Say it like this
At any given partial pressure of oxygen, the percentage saturation of haemoglobin is lower when carbon dioxide levels are higher
Notice what the shift does not do: at the high pressures found in the lungs both curves are still near the top, so loading up with oxygen is barely affected.
Why this is such a good design
Think about what the shift is responding to. The signal that moves the curve is carbon dioxide, and carbon dioxide is produced by respiration. So the tissues that are working hardest produce the strongest signal and get the biggest delivery. No nerves, no hormones, no control centre – the system regulates itself.
Where the blood is
pCO2
pH
Effect on the curve
What happens to oxygen
Alveoli in the lungs
Low
Higher
Curve sits to the left
Haemoglobin loads up and leaves nearly saturated
Resting tissue
Moderate
Slightly lower
Small shift right
Some oxygen released
Hard-working muscle
High
Lowest
Large shift right
Much more oxygen released to the cells
Left shift or right shift?
Shift
Caused by
Affinity
Meaning
Left
Foetal haemoglobin, low CO2, higher pH
Higher
Binds oxygen more readily; higher saturation at the same pO2
Right
High CO2, lower pH, exercise
Lower
Releases oxygen more readily; lower saturation at the same pO2
🧠
Right means release
Right shift = Release. Anything that says “this tissue is working hard” – more CO2, more acid, more heat – pushes the curve right and hands over more oxygen.
Foetal haemoglobin again. Carbon dioxide has less of an allosteric effect on foetal haemoglobin than on adult haemoglobin. So even in placental blood, which is rich in carbon dioxide, foetal haemoglobin keeps its high affinity and can still take oxygen from the mother.
Worked examples
WE 1
Explaining the Bohr shift
Explain how a high partial pressure of carbon dioxide reduces the affinity of haemoglobin for oxygen. (4 marks)
Step 1: the reaction
Carbon dioxide combines with water in the blood to form carbonic acid.
Step 2: the ions
The carbonic acid dissociates into hydrogencarbonate ions and hydrogen ions, so the pH of the blood falls.
Step 3: the protein
The hydrogen ions bind to haemoglobin and change its conformation.
Step 4: the effect
Its affinity for oxygen is reduced, so oxygen is released and the dissociation curve shifts to the right.
CO₂ → carbonic acid → H⁺ → haemoglobin changes shape → oxygen releasedfour marks, four links in the chain – do not jump from CO₂ straight to “oxygen is released”
WE 2
Reading the extra oxygen released
Using the graph above, calculate the difference in percentage saturation at a pO2 of 4 kPa and explain what this means for a respiring muscle. (3 marks)
Step 1: read both curves
Low CO₂: about 59% saturated. High CO₂: about 38% saturated.
Step 2: subtract59 − 38 = 21 percentage pointsStep 3: what it means
An extra 21% of the haemoglobin’s oxygen is unloaded at the same partial pressure, so a muscle producing a lot of carbon dioxide receives considerably more oxygen for respiration.
About 21 percentage points more oxygen releasedanswers within a percent or two of the graph reading are accepted, but you must show where you read from
WE 3
The Bohr shift during exercise
Explain the advantage of the Bohr shift to an athlete during a race. (3 marks)
Point 1: what the muscles produce
Working muscles respire rapidly and release a lot of carbon dioxide, so the pCO₂ in the surrounding blood is high.
Point 2: the shift
The curve shifts to the right, so haemoglobin has a lower affinity and dissociates from oxygen more readily at that partial pressure.
Point 3: the advantage
More oxygen is delivered to precisely the muscles that are working hardest, so aerobic respiration can continue and less lactate is produced.
The waste product itself triggers the extra oxygen deliverysay “the tissues that need it most” – the self-regulating point is usually the third mark
💡 Exam tips
Learn the chain in order: CO2 → carbonic acid → H+ → lower pH → haemoglobin changes shape → oxygen released.
Always describe a shift with the phrase at any given partial pressure of oxygen.
Right shift = lower affinity = more oxygen released. Never the other way round.
If a graph has two unlabelled curves, the one further right is the one with more CO2.
Note that loading in the lungs is hardly affected, because both curves are near 100% at high pO2.
Use hydrogen ions, not “acid”, when you explain what binds to the haemoglobin.
⚠ Common mistakes
Saying carbon dioxide takes oxygen’s place on the haem group. It works through hydrogen ions and a change in shape.
Saying the Bohr shift raises affinity. It lowers it.
Getting the direction wrong. More CO2 moves the curve right.
Saying pH increases when carbon dioxide rises. It falls; the blood becomes more acidic.
Writing that haemoglobin stops working. It still binds oxygen in the lungs; only unloading changes much.
Describing the shift without saying why it helps. The advantage – more oxygen to active tissue – is almost always worth a mark.
Up next: The Oxygen Dissociation Curve – the full story of why that line is S-shaped, and how to read every region of it with confidence.
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