the lesion site of pulmonary TB

[hpw]

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The lesion site of pulmonary TB usually localized to the apical and posterior segments
of the upper lobes, and the superior segments of the lower lobes.

but why?
is it about oxygen distribution in the lung?

 

[OveractiveBrain]

The lesion site of pulmonary TB usually localized to the apical and posterior segments
of the upper lobes, and the superior segments of the lower lobes.

but why?
is it about oxygen distribution in the lung?

Stand up. Which way does gravity pull your body?

Down.

Stand up. Which way does gravity pull your lungs?

Down

Now, its not so rigid as this, because obviously the diaphragm moves, but imagine the thorax as a rigid box. The lungs are pinned to the top. As gravity pulls the lung down, it kind of gets all scrunched at the bottom. Where its pinned at the top, the lungs get stretched out. This is why the apex of the lung gets ventilated better. With each respiration, the lung at the top is freer to move air. Better ventilation means higher oxygen tension at the apex.

Blood is a fluid, and it too is impacted by gravity.Thus, for very similar reasons, the base of the lungs are perfused better, and inherently, the apices are perfused worse.

Primary TB in middle lobes, reactivated TB in upper lobes

 

[ucladoc2b]

I was always under the impression that the Alveolar ventilation was actually lower at the apex vs. the base (West, Respiratory Physiology: The Essentials, 9th Ed, P 66) but the Ventilation/Blood Flow ratio (V/Q) is much higher (mostly due to lower blood flow). Hence, you see a higher PO2 at the apex and a better environment for TB.

 

[hpw]

thank you for all of your answering🙂