Monday, February 2, 2015

Why secondary tuberculosis affects the upper lobe?

There is an entity called ventilation perfusion ratio (V/Q)
meaning the degree of air entering the alveoli of lung and the corresponding pulmonary blood supply to the same. The normal value of the V/Q ratio is 0.8

Pulmonary blood capillaries are a low pressure system, with an average pressure of 25/8 mm of Hg. Out here comes the effect of gravity, pulmonary blood is unable to perfuse the upper lobes that well .

On the other hand, Air when it enters the lungs it enters the upper lobes better than the middle and lower lobes

This fact can be applied to
-
1.     Upper lobe has a higher V/Q ratio
2.     Middle lobe has an ideal V/Q ratio
3.     Lower lobe has a lower V/Q ratio

Therefore, mycobacterium tuberculosis being an obligate aerobe is able to survive in the upper lobes ( most notably the apex )

-M-

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