The Bohr Effect

The Bohr effect is the decrease in the affinity of haemoglobin for oxygen when the partial pressure of carbon dioxide (pCO₂) is high. On a graph, a higher pCO₂ shifts the oxygen dissociation curve to the right.How it happens: in respiring tissues, cells produce carbon dioxide. CO₂ diffuses into red blood cells, where the enzyme carbonic anhydrase converts it to carbonic acid, which dissociates into hydrogen ions (H⁺) and hydrogencarbonate ions. The H⁺ ions bind to haemoglobin, forming haemoglobinic acid. This changes the shape of haemoglobin, reducing its affinity for oxygen, so oxyhaemoglobin releases more oxygen.Why it is useful:• In actively respiring tissues, pCO₂ is high, so haemoglobin gives up oxygen more readily. The most active tissues, which produce most CO₂ and need most oxygen, receive the most.
• In the lungs, CO₂ diffuses out into the alveoli, so pCO₂ is low. Haemoglobin's affinity for oxygen rises, and it loads oxygen more readily.Worked example: at a pO₂ of 4 kPa, haemoglobin is 60% saturated when pCO₂ is low, but only 45% saturated when pCO₂ is high. So at the same pO₂, a further 60 − 45 = 15% of the maximum oxygen is released because of the Bohr effect.