Bohr effect: carbon dioxide, hydrogen ions and oxygen unloading
The Bohr effect is the fall in haemoglobin's affinity (how tightly it holds oxygen) as carbon dioxide and hydrogen ions (acid) rise, releasing more oxygen in active tissues.
Remember
When tissues are busy, extra carbon dioxide and hydrogen ions make haemoglobin hold oxygen less tightly, so more oxygen is released where it is needed.
Memory aid
BohR is Right
The capital R in BohR matches the word Right, a reminder that the Bohr effect shifts the curve to the right.
In the lungs, haemoglobin takes on oxygen, and in the tissues it gives that oxygen back. How tightly haemoglobin holds oxygen (its affinity for oxygen) changes with the conditions around it.
Working muscle makes a lot of carbon dioxide. In red cells, the enzyme carbonic anhydrase turns carbon dioxide into carbonic acid (the acid formed from carbon dioxide), which releases a hydrogen ion (H⁺) and lowers the local pH. The local blood is more acidic.
Haemoglobin can bind both carbon dioxide and hydrogen ions, and binding them lowers its affinity for oxygen. Haemoglobin therefore releases more oxygen, or unloads it, in the busiest tissues, so supply matches demand. The combined effect of raised carbon dioxide and hydrogen ions in lowering haemoglobin's affinity for oxygen is the Bohr effect.
On the oxygen dissociation curve (the graph of haemoglobin's oxygen saturation against oxygen pressure), the Bohr effect appears as a shift to the right. The P50 of the curve, the oxygen pressure at which haemoglobin is half saturated, rises, and affinity falls.
Raised temperature and raised 2,3-BPG (2,3-bisphosphoglycerate, a molecule made by red cells that also lowers affinity) shift the curve to the right in the same way, but the Bohr effect itself refers to carbon dioxide and hydrogen ions.
Key facts
- Cause
- Active tissues make a lot of carbon dioxide, which carbonic anhydrase turns into carbonic acid and a hydrogen ion, so the local pH falls.
- Effect on haemoglobin
- Haemoglobin holds oxygen less tightly when carbon dioxide and hydrogen ions are raised, and this is called the Bohr effect.
- Curve
- The Bohr effect shifts the oxygen dissociation curve to the right, which goes with a raised P50.
- Purpose
- Supply is matched to demand, because the busiest tissues unload the most oxygen.
- Other right shifts
- Raised temperature and raised 2,3-BPG also shift the curve to the right, but they are a separate effect and not the Bohr effect.
- Do not confuse
- The Haldane effect is the reverse relationship to the Bohr effect: the effect of oxygen on carbon dioxide carriage (oxygen changing carbon dioxide binding).
Exam traps
- More acidic conditions shift the oxygen dissociation curve to the right, not the left, because the extra hydrogen ions lower haemoglobin's affinity for oxygen. A leftward shift means a higher affinity, as when pH rises.
- Higher tissue temperature does reduce haemoglobin's affinity for oxygen, but that is a separate effect and not the Bohr effect, which describes carbon dioxide and hydrogen ions.
- Carbon dioxide does not compete with oxygen for the same binding site on haemoglobin. Its effect on affinity is indirect, because it binds elsewhere on haemoglobin and works through the hydrogen ions that carbonic anhydrase makes.
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