- The oxygen-hemoglobin dissociation curve illustrates the relationship between the partial pressure of oxygen (pO2) and hemoglobin saturation.
- A rightward shift of this curve indicates that hemoglobin has a decreased affinity for oxygen, facilitating oxygen release to the tissues.

- Among the given options, elevated hydrogen ion concentration (lower pH) is the correct factor causing a rightward shift.
- This phenomenon is known as the Bohr effect, where increased H⁺ ions bind to hemoglobin and reduce its oxygen affinity.
- As a result, oxygen is more readily released in metabolically active tissues that have higher acidity.

In contrast, the other factors generally cause a leftward shift (increased oxygen affinity):
- Decreased 2,3-Bisphosphoglycerate (2,3-BPG) levels: 2,3-BPG binds to hemoglobin and lowers oxygen affinity; a decrease in 2,3-BPG increases affinity, causing a left shift.
- Reduced carbon dioxide levels: Low CO2 reduces H+ concentration, increasing hemoglobin’s oxygen affinity.
- Lowered body temperature: Cooler temperatures increase hemoglobin’s affinity for oxygen.

Therefore, the key point is that increased H⁺ concentration (lower pH) lowers hemoglobin’s affinity for oxygen, causing a rightward shift in the dissociation curve and promoting oxygen unloading in tissues needing it most.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th Edition, Chapter 35: Oxygen Transport