During exercise, skeletal muscle tissues experience a decrease in local partial pressure of oxygen. According to the oxygen dissociation curve, why does hemoglobin release oxygen more readily under these conditions?

• A. Reduced oxygen binding to one heme group lowers hemoglobin affinity and promotes additional oxygen release
• B. Lower oxygen levels cause hemoglobin to gain extra oxygen-binding sites
• C. Oxygen release occurs only after all four oxygen molecules detach simultaneously
• D. Hemoglobin affinity for oxygen increases as tissue oxygen levels decrease

Answer: (A)