Correct Response: A. Loading and unloading of oxygen from hemoglobin is sensitive to the pH of the blood. As blood becomes more acidic due to the release of carbon dioxide as a byproduct of cellular respiration, the affinity of hemoglobin for oxygen decreases, and oxygen is released into the surrounding tissues. The affinity of hemoglobin for oxygen does increase after the initial oxygen molecule binds (B), but this mechanism does not control the loading or unloading of the initial oxygen molecule onto hemoglobin. Blood arriving at the lungs does have a lower PO2 than the air inside the lungs (C), causing oxygen to diffuse into the blood from the lungs; however, this does not explain the mechanism by which hemoglobin loads and unloads oxygen. Although the heart rate does increase in response to increased levels of carbon dioxide in the blood (D), this does not describe the mechanism by which hemoglobin loads and unloads oxygen.
