Final answer:
The correct order from highest to lowest Po2 is alveolar air, pulmonary veins, and then pulmonary artery, corresponding to option D: 3,1,2.
Step-by-step explanation:
The question asks to arrange the following in order from highest to lowest partial pressure of oxygen (Po2): (1) Po2 of pulmonary veins, (2) Po2 of pulmonary artery, (3) Po2 of alveolar air.
Considering the physiology of oxygen transport in the blood and gas exchange in the lungs, the alveolar air has the highest Po2, followed by the pulmonary veins, which carry oxygenated blood back to the heart, and finally, the pulmonary artery, which carries deoxygenated blood to the lungs from the heart. Therefore, the answer to the question is option D: 3,1,2, which means alveolar air, pulmonary veins, and then pulmonary artery in descending order of Po2.