The Bohr Effect

The Bohr effect (Bohr shift) is the decrease in the affinity of haemoglobin for oxygen when the partial pressure of carbon dioxide (pCO₂) is high. It causes the oxygen dissociation curve to shift to the right.How it happens:• carbon dioxide from respiration enters the erythrocytes, where the enzyme carbonic anhydrase catalyses its reaction with water to form carbonic acid
• carbonic acid dissociates into hydrogen ions (H⁺) and hydrogencarbonate ions
• the H⁺ ions bind to haemoglobin, changing its shape and reducing its affinity for oxygen, so oxyhaemoglobin releases more oxygenWhen the curve shifts right, haemoglobin is less saturated at any given pO₂. This is useful in two places:• in actively respiring tissues, such as exercising muscle, pCO₂ is high, so more oxygen is unloaded exactly where it is most needed
• in the lungs, carbon dioxide diffuses out into the alveoli, pCO₂ is low, so haemoglobin's affinity rises and it loads oxygen readilyThe more active a tissue is, the more carbon dioxide it produces, so the more oxygen it receives. Lactic acid produced during intense exercise also lowers the pH, and a rise in temperature also shifts the curve to the right.Worked example: at a pO₂ of 4 kPa, haemoglobin is 60% saturated at normal pCO₂ but only 40% saturated at high pCO₂. So at high pCO₂ an extra 20% of the maximum oxygen is released to the tissue.