Bohr is carbon dioxide affecting oxygen carriage; Haldane is oxygen affecting carbon dioxide carriage
Mnemonic
“Bohr is about oxygen, Haldane is about carbon dioxide”, and each helps the other:
- Bohr effect - carbon dioxide and acid reduce haemoglobin’s affinity for oxygen, so the curve shifts right and oxygen is released in metabolically active tissue
- Haldane effect - deoxygenated haemoglobin carries carbon dioxide better, so unloading oxygen in the tissues helps pick up carbon dioxide, and oxygenating in the lung helps release it
“Bohr unloads oxygen where carbon dioxide is high; Haldane loads carbon dioxide where oxygen is low.” They are two halves of the same elegant arrangement.
Carbon dioxide is carried three ways: about 70 per cent as bicarbonate, 23 per cent as carbamino compounds bound to haemoglobin, and 7 per cent dissolved.
The Haldane effect explains a clinical trap: giving high concentration oxygen in chronic type 2 respiratory failure raises carbon dioxide partly by this mechanism, not only through loss of hypoxic drive and worsened V/Q matching.
Expansion
- Bohr effect - a rise in carbon dioxide or fall in pH reduces haemoglobin’s affinity for oxygen, shifting the dissociation curve right and promoting oxygen unloading in metabolically active tissue
- Haldane effect - deoxygenated haemoglobin carries more carbon dioxide, both as carbamino compounds and by buffering hydrogen ions, so releasing oxygen in the tissues promotes carbon dioxide uptake
The two are reciprocal and mutually reinforcing:
- In the tissues: carbon dioxide enters, causing a Bohr shift that releases oxygen; the resulting deoxyhaemoglobin then takes up more carbon dioxide by the Haldane effect
- In the lungs: oxygen binds, causing the Haldane effect to release carbon dioxide; losing carbon dioxide then raises pH and shifts the curve left, aiding oxygen loading
The Haldane effect accounts for about half of carbon dioxide carriage in normal arteriovenous exchange, which is more than most people expect.
It also explains a clinical hazard: giving high concentration oxygen in chronic obstructive pulmonary disease raises carbon dioxide partly by the Haldane effect, not only by reducing hypoxic drive.