The most likely option functioning improperly in this scenario is the alveoli. The alveoli are the tiny air sacs in the lungs where gas exchange occurs, allowing oxygen to enter the blood and carbon dioxide to be expelled. If the alveoli are not functioning properly, it can lead to low blood oxygen levels even if blood is circulating normally.
The other options, such as the pulmonary vein, aorta, and capillaries, would primarily affect the distribution and circulation of blood rather than the actual oxygen uptake process in the lungs.