Bohr effect

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Hemoglobin Dissociation Curve. Dotted red line corresponds with shift to the right caused by Bohr effect

Bohr effect is a property of hemoglobin first described in 1904 by the Danish physiologist Christian Bohr (father of physicist Niels Bohr), which states that in the presence of carbon dioxide, the oxygen affinity for dissociation of respiratory pigments, such as hemoglobin, decreases; because of the Bohr effect, an increase in blood carbon dioxide level or a decrease in pH causes hemoglobin to bind to oxygen with less affinity.

This effect facilitates oxygen transport as hemoglobin binds to oxygen in the lungs, but then releases it in the tissues, particularly those tissues in most need of oxygen. When a tissue's metabolic rate increases, its carbon dioxide production increases. The carbon dioxide is quickly converted into bicarbonate molecules and acidic protons by the enzyme carbonic anhydrase:

CO₂+ H₂O $\rightleftharpoons$ H⁺ + HCO₃⁻

This causes the pH of the tissue to decrease, and so increases the dissociation of oxygen from hemoglobin to the tissue, allowing the tissue to obtain enough oxygen to meet its demands (and raise its blood pH).

The dissociation curve shifts to the right when carbon dioxide or hydrogen ion concentration is increased. This facilitates increased oxygen dumping. This makes sense because increased CO₂ concentration and lactic acid build-up occur when the muscles need more oxygen. Changing hemoglobin's oxygen affinity is the body's way of adapting quickly to this problem.

The lactic acid formed in muscle does not exist as such, since the pK_a of the -COOH group in lactic acid is +2.86. The pH of muscle tissue is about 7.4, so according to the Henderson–Hasselbalch equation since pH >> pK_a, lactic acid will be present not as H₃C-CHOH-COOH but only as the ionized form H₃C-CHOH-COO^-.

In the Hiroshima variant hemoglobinopathy the Bohr effect is diminished so the hemoglobin has a higher affinity for oxygen and tissue may suffer minor oxygen starvation during high work.