The Bohr shift is the decrease in haemoglobin’s affinity for oxygen when carbon dioxide concentration increases. It causes haemoglobin to release more oxygen in actively respiring tissues where oxygen is most needed.
How does the Bohr shift work?
- Actively respiring cells use oxygen and produce carbon dioxide during aerobic cell respiration.
- Carbon dioxide diffuses from the cells into the blood, increasing the blood’s carbon dioxide concentration and lowering its pH.
- Carbon dioxide and the associated increase in hydrogen ion concentration interact with haemoglobin, changing its conformation.
- This conformational change reduces haemoglobin’s affinity for oxygen.
- Oxyhaemoglobin therefore dissociates, releasing oxygen that diffuses into the respiring tissues.
On an oxygen dissociation curve, this decreased affinity appears as a shift to the right. At the same partial pressure of oxygen, haemoglobin has a lower percentage saturation when carbon dioxide concentration is higher.
| Condition | Effect on haemoglobin and oxygen transport |
|---|---|
| High carbon dioxide and lower pH in active tissues | Lower oxygen affinity; right shift; increased oxygen unloading |
| Low carbon dioxide and higher pH in the lungs | Higher oxygen affinity; movement to the left; increased oxygen loading |
For example, exercising muscle produces more carbon dioxide than resting muscle. The resulting Bohr shift allows more oxygen to be unloaded from haemoglobin, supporting the muscle’s increased rate of aerobic cell respiration.
A common misconception is that the Bohr shift means haemoglobin carries more oxygen. It actually means that, under high-carbon-dioxide conditions, haemoglobin binds oxygen less strongly and releases a greater proportion of it.
Exam technique
This is AHL content in B3.1 Gas exchange. For an “explain” question, link the full sequence: increased carbon dioxide → lower pH or altered haemoglobin conformation → reduced oxygen affinity → increased dissociation and oxygen delivery. If given oxygen dissociation curves, identify the curve shifted to the right as the one representing higher carbon dioxide concentration.
