Introduction

The oxygen-hemoglobin dissociation curve shows how readily hemoglobin binds to and releases oxygen under varying physiological conditions. A shift in this curve to the left or right represents changes in hemoglobin’s oxygen affinity.

What a Left Shift Means

• A left shift indicates increased affinity of hemoglobin for oxygen.

• Hemoglobin binds oxygen more tightly, meaning it loads oxygen more easily in the lungs but unloads it less easily in the tissues.

• This shift typically occurs in environments where oxygen release to tissues is less urgently needed or in preparation for oxygen loading in the lungs.

Causes of a Left Shift

• Increased pH (alkalosis): Fewer hydrogen ions stabilize hemoglobin to hold oxygen more tightly.

• Decreased pCO2: Less carbon dioxide reduces acidity, favoring oxygen binding.

• Decreased temperature: Cooler temperatures promote stronger Hb-O2 binding.

• Decreased 2,3-Bisphosphoglycerate (2,3-BPG): Lower 2,3-BPG reduces competition at hemoglobin, increasing oxygen affinity.

Evaluating the Answer Choices

Summary Table

Conclusion

When the oxygen-hemoglobin dissociation curve shifts to the left, it represents:

(4) More affinity for oxygen

Hemoglobin binds oxygen more tightly, enhancing oxygen loading in the lungs but potentially reducing oxygen release in peripheral tissues.