Which statement best describes the oxygen dissociation curve?

Which statement best describes the oxygen dissociation curve?

• A. Hb has equal affinity irrespective of PO2
• B. At low PO2 Hb binds CO2
• C. At high PO2 Hb binds O2 to form oxyhaemoglobin
• D. Hb cannot bind O2 in tissues

Answer: (C)

Understanding how oxygen binding to hemoglobin changes with partial pressure of oxygen helps explain why this curve looks the way it does. The oxygen dissociation curve describes how saturated hemoglobin is with O2 at different PO2 levels. In the lungs, PO2 is high, so hemoglobin readily binds O2 and forms oxyhaemoglobin. This loading process is why the statement that at high PO2 Hb binds O2 to form oxyhaemoglobin is the best description of what the curve shows. The curve’s sigmoidal shape arises from cooperative binding—when one oxygen molecule binds, it increases the affinity for the next ones, promoting rapid loading as PO2 rises. The other ideas don’t fit the curve: affinity isn’t the same at all PO2 levels, because binding becomes easier as oxygen binds; the curve is about O2 binding, not CO2 binding; and hemoglobin does not lose the ability to bind O2 in tissues—it's where O2 is released from Hb, not where binding to Hb is prohibited.

Understanding how oxygen binding to hemoglobin changes with partial pressure of oxygen helps explain why this curve looks the way it does. The oxygen dissociation curve describes how saturated hemoglobin is with O2 at different PO2 levels. In the lungs, PO2 is high, so hemoglobin readily binds O2 and forms oxyhaemoglobin. This loading process is why the statement that at high PO2 Hb binds O2 to form oxyhaemoglobin is the best description of what the curve shows. The curve’s sigmoidal shape arises from cooperative binding—when one oxygen molecule binds, it increases the affinity for the next ones, promoting rapid loading as PO2 rises.

The other ideas don’t fit the curve: affinity isn’t the same at all PO2 levels, because binding becomes easier as oxygen binds; the curve is about O2 binding, not CO2 binding; and hemoglobin does not lose the ability to bind O2 in tissues—it's where O2 is released from Hb, not where binding to Hb is prohibited.