How does plasma PCO2 affect hemoglobin's affinity for oxygen in active muscles?

• A. Increased PCO2 decreases affinity for oxygen
• B. Decreased PCO2 increases affinity for oxygen
• C. Plasma PCO2 has no effect on its affinity
• D. Increased PCO2 increases affinity for oxygen

Answer: (A)

The correct answer is that increased PCO2 decreases affinity for oxygen. This phenomenon is part of the Bohr effect, where an increase in carbon dioxide concentration (PCO2) in tissues, such as active muscles, leads to a decrease in the pH of the blood (due to the formation of carbonic acid).

As PCO2 rises, hemoglobin's structure is altered, resulting in a lower affinity for oxygen. This reduction in affinity promotes the release of oxygen from hemoglobin, which is beneficial because actively respiring tissues require more oxygen to support increased metabolic activity during exercise. The increased PCO2 signals that tissues are undergoing high metabolism and need more oxygen for cellular respiration, enhancing oxygen delivery where it is most needed.

This adaptive mechanism maximizes oxygen unloading in tissues with elevated carbon dioxide levels, facilitating efficient gas exchange that is critical during periods of increased physical demand.