How does a right shift vs left shift of the oxyhemoglobin dissociation curve affect oxygen affinity?

• A. Right shift increases affinity; left shift decreases
• B. Right shift does not change affinity; left shift increases
• C. Right shift increases affinity; left shift decreases
• D. Right shift decreases affinity; left shift increases

Answer: (D)

The key idea is that shifting the oxyhemoglobin dissociation curve reflects changes in how tightly hemoglobin binds oxygen. A rightward shift means hemoglobin has a lower affinity for O2, so at the same partial pressure of oxygen, hemoglobin releases O2 more readily to tissues. This happens with factors like higher temperature, higher CO2, lower pH (Bohr effect), and increased 2,3-BPG—conditions common in active tissues needing more oxygen.

A leftward shift means hemoglobin binds O2 more tightly, increasing affinity, so it holds onto O2 more at a given PO2 and releases it less readily to tissues. This occurs with lower temperature, higher pH, lower CO2, and decreased 2,3-BPG, such as in the lungs or in fetal blood where loading of O2 is favored.

So in terms of oxygen affinity: a right shift decreases affinity; a left shift increases affinity.