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-
-M-
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