Which condition causes a rightward shift of the simplified oxyhemoglobin dissociation curve?

• A. Increased CO2
• B. Decreased CO2
• C. Decreased temperature
• D. Increased pH

Answer: (A)

A rightward shift means hemoglobin releases oxygen more readily, i.e., its affinity for O2 decreases. When CO2 rises, it reacts with water to form carbonic acid, which dissociates to bicarbonate and hydrogen ions, lowering pH in the blood and tissues. That acidification, known as the Bohr effect, reduces Hb’s affinity for oxygen, pushing the curve to the right. CO2 can also bind directly to hemoglobin (carbaminohemoglobin), further decreasing oxygen affinity. So increased CO2 shifts the curve to the right.

In contrast, conditions that lower CO2, raise pH (alkalosis), or lower temperature increase hemoglobin’s affinity for oxygen and shift the curve to the left, which is why those factors don’t match the rightward shift.