Tip

• The binding of O2 molecules to hemoglobin in the lungs has two consequences, known as the Haldane effect:
  • The affinity of hemoglobin for CO2 is decreased, resulting in unloading of CO2 from hemoglobin (this accounts for a small percentage of overall CO2 in the blood and is not pictured above).
  • The acidity of the hemoglobin molecule is increased; in response, protons (H+ ions) are released from the hemoglobin binding sites.
• The H+ ions combine with bicarbonate ions (the primary form of CO2 in the blood) in the lungs to facilitate the production of water (H2O) and CO2. The CO2 is then transferred to the alveoli and expired while oxygen is taken up by hemoglobin.
• In the peripheral tissues, high levels of CO2 create an increase in ambient acidity that shifts the hemoglobin dissociation curve to the right and facilitates the unloading of O2 (Bohr effect). The CO2 (and water) are converted into H+ and HCO3−. The H+ ions are carried by hemoglobin while the HCO3− is transferred to the plasma for transport back to the lungs.

Bohr effect

The O2 affinity of Hb is inversely proportional to the CO₂ content and H⁺ concentration of blood. High CO₂ and H⁺ concentrations (from tissue metabolism) cause decreased affinity for O2 → O2 that is bound to Hb is released to tissue (the O2-Hb dissociation curve is shifted to the right). HbO2 + H⁺ ⇄ H⁺Hb + O2 HbO2 + CO₂ ⇄ Hb-COO- + H⁺ + O2

Haldane effect

The CO₂ affinity of Hb is inversely proportional to the oxygenation of Hb. When Hb is deoxygenated (typically in peripheral tissue), uptake of CO₂ is facilitated. When Hb is oxygenated (in high pO2, for example, in the lungs): Oxygenated Hb has a decreased affinity for CO₂ → CO₂ that is bound to Hb is released in the pulmonary arteries to diffuse into the alveoli (the O2-Hb dissociation curve is shifted to the left). Hb releases bound H⁺ → ↑ H⁺ shifts equilibrium to CO₂ production (see equation above) → CO₂ is exhaled in lungs

Mnemonic

Bohr for body, HaLdane for lung