Correct Response: B. The structure of lungs in birds is different from that of mammals. In birds, air flows in one direction through parabronchioles, where gas exchange with capillaries occurs. Each breath completely exchanges air in the lungs. This is more efficient than the mammalian lung structure, in which air flow is bidirectional and gas exchange occurs in alveoli. Birds and mammals both use negative-pressure breathing (A) in which air is drawn into the body by the expansion of the rib cage. Although birds at high altitudes would likely have a lower PO2 in their lungs than animals at lower altitudes (C), this would tend to make oxygen exchange with the blood more difficult and is a challenge birds must overcome. Hemoglobin is responsible for transporting oxygen in birds as well as in mammals (D), although the hemoglobin produced by high-altitude birds may have a greater affinity for oxygen.
