Why is the intrapleural pressure more negative in the apex of the lung compared to the hilum?

[sbm1292987]

I realise this is due to gravity, but I don't really understand the mechanism. I read something about the lung base weighing more due to increased blood flow or something like that, but I don't understand how that affects the intrapleural pressure

Any help would be appreciated!

EDIT: made a mistake in the title - it's meant to say "apex compared to the base"

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[slz1900]

Think about what establishes intrapleural pressure. The lungs want to collapse and the chest cavity wants to expand -> negative pressure between them in intrapleural space.

The effect of gravity at the apex can be interpreted as an additional collapsing force on the lungs. Not only do the alveoli want to collapse because of intrinsic properties, gravity is also pushing on alveoli at the apex, resulting in a greater collapsing force and a more negative intrapleural pressure.

 

[zhopv10]

Quick question on this: do we assume that the chest wall expansion force is unaffected by gravity? Because it seems that unless we do the collapsing force would be increased at the apex as you said but the expansion force would be decreased leading to variable effect on intrapleural pressure?

*edit: not disagreeing, what you said, its totally true, I've just always wondered about the above in regards to it.


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[slz1900]

Quick question on this: do we assume that the chest wall expansion force is unaffected by gravity? Because it seems that unless we do the collapsing force would be increased at the apex as you said but the expansion force would be decreased leading to variable effect on intrapleural pressure?

*edit: not disagreeing, what you said is totally true, I've just always wondered about the above in regards to it.


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The chest wall wants to expand outward, not upward, so I would say that gravity's effect is minimal. The reason gravity can exert an effect on lung physiology is because the lungs are soft and can collapse on themselves a bit. The ribs won't be compressed due to the effects of gravity because they are rigid structures supported by musculature.

(of course gravity will effect everything and you could quantify it mathematically if you knew the compressibility of bone, muscles, etc but practically it only matters for the lungs)

 

[zhopv10]

Solid, makes sense!


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[sbm1292987]

Think about what establishes intrapleural pressure. The lungs want to collapse and the chest cavity wants to expand -> negative pressure between them in intrapleural space.

The effect of gravity at the apex can be interpreted as an additional collapsing force on the lungs. Not only do the alveoli want to collapse because of intrinsic properties, gravity is also pushing on alveoli at the apex, resulting in a greater collapsing force and a more negative intrapleural pressure.

This might be a really stupid question, but how come you don't consider the gravitational force pulling on the base too?

My best guess: is it because of the "direction" of gravity, since gravity on the apex acts "inwards" whereas gravity on the apex base "outwards"?

 

[zhopv10]

This might be a really stupid question, but how come you don't consider the gravitational force pulling on the base too?

My best guess: is it because of the "direction" of gravity, since gravity on the apex acts "inwards" whereas gravity on the apex base "outwards"?

Ya the gravity is pulling away from the inwardly collapsing pressure in the base. (Opposite of the apex).


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[sbm1292987]

Ya the gravity is pulling away from the inwardly collapsing pressure in the base. (Opposite of the apex).


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Perfect, thanks!

 

[chillaxbro]

I always thought "gravity" was a BS answer. How is ~10 inches of height going to make that much of a difference via only gravity?

 

[slz1900]

I always thought "gravity" was a BS answer. How is ~10 inches of height going to make that much of a difference via only gravity?

Fill up a water balloon and set it on the floor. The top will squish down on the bottom and it'll flatten a little, due to gravity. That's analogous to what happens in the lungs.

 

[chillaxbro]

Fill up a water balloon and set it on the floor. The top will squish down on the bottom and it'll flatten a little, due to gravity. That's analogous to what happens in the lungs.

So its not a matter of "more" gravity on the lower lobes. The stuff just gets pulled down there? What's the stuff?

 

[slz1900]

So its not a matter of "more" gravity on the lower lobes. The stuff just gets pulled down there? What's the stuff?

Right it's not that there's more gravity, just that gravity's effect on the apex is dispersed . Back to the round ballon, let's say it is mostly full and you set it on a surface and start to push down on it lightly from above. The top will flatten but the shape of the rest of the balloon doesn't change much at first (unless you push more firmly) because the force at the top is transmitted throughout it. Don't know if that imagery works but I think it's a good way of visualizing it.

 

[Tappinfool66]

I have a pretty different understanding of this topic, or at least a different way of imagining it. I figured I would post in case anyone else was still confused.

It's important to remember the difference between intrapleural pressure and alveolar pressure. The alveoli are open to the environment so alveolar pressure (i.e. the pressure inside the lungs) is really the same as atmospheric pressure. Intrapleural pressure is the pressure of the air in the pleural cavity in which the lung sits (the space between the lung tissue and the chest wall).

It's also important to note that no pressures are actually negative. Pressure can't be negative. But we refer to intrapleural pressure as "negative," which just means that it is less than alveolar pressure.

So, the intrapleural pressure around the apical alveoli is less ("more negative") than the intrapleural pressure around basal alveoli. Why? From what I understand, the answer is gravity. Gravity pulls the air molecules in the intrapleural space down from the apical area toward the basal area, making them settle in the basal area. The effect is:

-a higher ("less negative") basal intrapleural pressure. This higher pressure exerts a larger force on the alveoli, causing basal alveoli to close.
and
-a lower ("more negative") apical intrapleural pressure. This lower pressure exerts less force on the apical alveoli, causing them to be more open.

This is important because the differences in intrapleural pressure leads to differences in perfusion and that, in combination with differences in regional blood flow, leads to v/q mismatch in the lungs (physiologic dead space in the apices, shunt in the bases).

I think that answers OP's main question. If anyone has any other questions or if anything I've said is incorrect, feel free to let me know.

 

[Entadus]

I have a pretty different understanding of this topic, or at least a different way of imagining it. I figured I would post in case anyone else was still confused.

It's important to remember the difference between intrapleural pressure and alveolar pressure. The alveoli are open to the environment so alveolar pressure (i.e. the pressure inside the lungs) is really the same as atmospheric pressure. Intrapleural pressure is the pressure of the air in the pleural cavity in which the lung sits (the space between the lung tissue and the chest wall).

It's also important to note that no pressures are actually negative. Pressure can't be negative. But we refer to intrapleural pressure as "negative," which just means that it is less than alveolar pressure.

So, the intrapleural pressure around the apical alveoli is less ("more negative") than the intrapleural pressure around basal alveoli. Why? From what I understand, the answer is gravity. Gravity pulls the air molecules in the intrapleural space down from the apical area toward the basal area, making them settle in the basal area. The effect is:

-a higher ("less negative") basal intrapleural pressure. This higher pressure exerts a larger force on the alveoli, causing basal alveoli to close.
and
-a lower ("more negative") apical intrapleural pressure. This lower pressure exerts less force on the apical alveoli, causing them to be more open.

This is important because the differences in intrapleural pressure leads to differences in perfusion and that, in combination with differences in regional blood flow, leads to v/q mismatch in the lungs (physiologic dead space in the apices, shunt in the bases).

I think that answers OP's main question. If anyone has any other questions or if anything I've said is incorrect, feel free to let me know..

This gas laws/thermodynamics explanation makes the most sense to me 👍

All this talk of the chest wall or lungs being "affected more" by gravity has me
In my understanding the solid portion of the body is being supported by the legs touching the ground "normal force" ... but gravity will tend to pull the gas particles downward... The gas at the base will tend towards higher density; the particles at the base of the container will have higher energy states the increased number of particles would result in a higher # of collisions which would increase the basal pressure (P = nRT/V). (apex pressure will be lower by the same logic)

 

[slz1900]

This gas laws/thermodynamics explanation makes the most sense to me 👍

All this talk of the chest wall or lungs being "affected more" by gravity has me
In my understanding the solid portion of the body is being supported by the legs touching the ground "normal force" ... but gravity will tend to pull the gas particles downward... The gas at the base will tend towards higher density; the particles at the base of the container will have higher energy states the increased number of particles would result in a higher # of collisions which would increase the basal pressure (P = nRT/V). (apex pressure will be lower by the same logic).

I thought intra-alveolar pressure was equal throughout the lung.