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 )


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