Why does Tuberculosis tend to reactivate in upper lung lobes?

Mycobacterium tuberculosis is an obligate aerobe and only survives in oxygenated areas.

Exact reason for TB’s propensity to reactivate in the upper lobes is not clear but two proposed rationales:

1. Relatively higher oxygen tension compared to lower lobes (The lower lobes have better perfusion and therefore better V/Q match => greater gas exchange and more CO2 content compared to upper lobes)
2. Impaired lymphatic drainage in upper lung lobes reduces the ability of the immune system to mount a response to the pathogen and clear it

References

1. Allen EA. Tuberculosis and other mycobacterial infections of the lung. In: Thurlbeck WM, Churlbeck AM, eds. Pathology of the lung. 2nd ed. New York, NY: Thieme Medical, 1995; 229-265.
2. Goodwin RA, DesPrez RM. Apical localization of pulmonary tuberculosis, chronic pulmonary histoplasmosis, and progressive massive fibrosis of the lung. Chest 1983; 83:801-80
3. Dolin, [edited by] Gerald L. Mandell, John E. Bennett, Raphael (2010). Mandell, Douglas, and Bennett’s principles and practice of infectious diseases (7th ed.). Philadelphia, PA: Churchill Livingstone/Elsevier. pp. Chapter 250.