AAMC Q - why does puncturing pleural tissue prevent the lung from expanding?

[mrh125]

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this was on a practice AAMCs and im still confused. iirc pleural tissue is associated with the wall of the chest and diapraghm tissue and if that's what puncturing pleural tissue does what would puncturing visceral tissue (inside lung) do? cause it to collapse?

edit: it said right pleural

 

[Cmdr_Shepard]

http://en.m.wikipedia.org/wiki/Pneumothorax

 

[Blueprint MCAT Tutor]

You expand your lungs with negative pressure - you suck inwards and the lower pressure inside the lungs causes air to flow in from the Patm outside.

If there's a hole in the lining, then when you try to suck in, you don't actually expand the lungs, you just get air flowing through the hole into the pleural space.

Imagine the following as a very rough analogy:

You cover your mouth with a balloon (or plastic wrap or whatever). You then try to breathe in. If the balloon is intact, that inward/sucking motion would cause the balloon to expand into your mouth.

If there were a hole in the balloon, then instead of expanding the balloon, air would just flow through the hole and the balloon would be limp.

The balloon is like the lung. If it's intact, then the sucking action from your diaphragm makes it expand. If there's a hole in the lining, then it doesn't expand.

Hope that helps!

b.

 

[Czarcasm]

I wish people would warn others before asking AAMC exam questions. It ruins it for the rest of us who haven't taken it yet. There's specific AAMC threads for this reason.

 

[Arayh]

I want to stress that the diaphragm doesn't suck in air, but rather, as it contracts and pulls downward, (along with the sternocleidomastoid, external intercostal, scalene muscles), it increases the thoracic cavity size which in turn decreases the alveolar pressure inside the lungs to a value slightly below atmospheric pressure (around 0.8-1 mmHg less). This decrease in pressure is what drives air into your lungs, as air will flow from high to low pressure.

Furthermore, your pleural cavity, formed by the parietal and visceral pleura contains pleural fluid which is "unexpandable and uncompressible" and maintains the pressure gradient, as well as allows frictionless movement during inhalation and exhalation. The pleural cavity has a constant "negative" (below atmospheric) pressure of between 5-8 mmHg depending on inhalation vs. exhalation phases.

When you puncture the parietal and/or visceral pleura, an influx of air between the chest wall and the lung equalizes the gradient between pleura pressure and barometric pressure and collapses the lung. However, due to the anatomic separation of the left and right pleura, only one lung will usually deflate at a time due to a pneumothorax.

You expand your lungs with negative pressure - you suck inwards and the lower pressure inside the lungs causes air to flow in from the Patm outside.

If there's a hole in the lining, then when you try to suck in, you don't actually expand the lungs, you just get air flowing through the hole into the pleural space.

Imagine the following as a very rough analogy:

You cover your mouth with a balloon (or plastic wrap or whatever). You then try to breathe in. If the balloon is intact, that inward/sucking motion would cause the balloon to expand into your mouth.

If there were a hole in the balloon, then instead of expanding the balloon, air would just flow through the hole and the balloon would be limp.

The balloon is like the lung. If it's intact, then the sucking action from your diaphragm makes it expand. If there's a hole in the lining, then it doesn't expand.

Hope that helps!

b.