A right shift on the oxygen affinity curve would result in what change to P50?

• A. Decreased P50
• B. No change
• C. P50 is undefined
• D. Increased P50

Answer: (D)

A rightward shift of the hemoglobin-oxygen dissociation curve indicates decreased affinity of hemoglobin for oxygen. With lower affinity, hemoglobin holds onto O2 less tightly and requires a higher partial pressure of oxygen to reach 50% saturation, so the P50 value increases. P50 is the PO2 at which hemoglobin is 50% saturated; a higher P50 means oxygen affinity is reduced and unloading at tissues is favored. Factors that cause a right shift include higher temperature, higher CO2, lower pH (the Bohr effect), and increased 2,3-BPG, all promoting easier O2 release. So the change associated with a right shift is an Increased P50.