A 41-year-old man originally from Vietnam presents to his primary care physician with a 6-week history of night sweats, low-grade fevers, and a productive cough with occasional blood-streaked sputum. He has no significant medical history and has never been tested for HIV. His HIV test today is negative. Chest radiograph shows a cavitary lesion in the right upper lobe. Sputum acid-fast bacilli (AFB) smear is positive. The attending physician explains that M. tuberculosis is an obligate aerobe that replicates best in high-oxygen environments, and asks the student why reactivation tuberculosis characteristically involves the upper lung zones.
Which of the following physiological features of the lung apex best explains this distribution?