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My OL biology textbook says that one of the functions of the pleural fluid between lungs and pleural membrane is that it ensures no air between lungs and rib cage.

But I wonder why air there would cause any problems. I was told that it'd build up there and exert pressure on the lungs but why would it build up there in first place, couldn't it just go out through the mouth or nose? And in case that's really the answer, how does it manage to prevent any oxygen or other gases from getting into the fluid, after all, the pleural fluid touches the pleural membranes cells which require oxygen, won't oxygen diffuse from around them into the fluid?

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  • $\begingroup$ having air or gas bubbles interacting with the intercostal muscles would be outside of the bodies air cavities which are gas control systems, free oxygen in the body is quite a reactive substance, having air bubbles forming outside of the lungs would be far from peak performance. $\endgroup$ – aliential May 6 '20 at 15:35
  • $\begingroup$ The pleural membranes help the lungs to move without friction, they are like water in between glass, the hydrostatic pressure is a low friction system. If you are running with bubbles outside your lungs, it would tend to move around and upwards, it would interact with the muscle tissues and the nerves, the intercostal are well innervated. $\endgroup$ – aliential May 6 '20 at 15:50
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The pleural cavity and the airway are separate compartments; there is no anatomical path between them.

enter image description here (Image source: https://upload.wikimedia.org/wikipedia/commons/0/0d/2313_The_Lung_Pleurea.jpg. Attribution: OpenStax College / CC BY (https://creativecommons.org/licenses/by/3.0))

Thus air in the pleural cavity would not go out through the mouth or nose. But it would make breathing difficult. How?

In a normal human being, the chest wall and diaphragm move outwards to bring about inspiration. Remember, though, that they are separated from the lung by pleural fluid. Since pleural fluid (like most liquids) does not compress or distend to any significant degree, the outward movement of the chest wall and diaphragm tends to create a negative pressure in the pleural space. This in turn expands the lungs. The reverse happens during expiration to increase pleural pressure and contract the lungs. In a sense, the pleural fluid 'couples' the movement of the chest wall to movement of the lung.

This coupling action would be lost with air, which is highly compressible and distensible. The bulk modulus of water (the main constituent of pleural fluid) is 2.2 GPa: about two million times the isothermal bulk modulus of air! With air in the pleural cavity, chest wall and diaphragm movement would do nothing to change lung volume. Rather, the air would happily accommodate to the new volume of the pleural cavity, without any pressure change.

As far as the diffusion of oxygen into pleural fluid is concerned, I believe it does happen (similar to what happens in extracellular fluid in any other location). However, I could not find any data on pleural fluid oxygen tensions specifically. For perspective, the interstitial fluid oxygen tension in different human tissues ranges from 3 to 7 kPa. I'd expect it to be higher in pleural fluid because of its close proximity to alveoli.

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Pleural fluid is secreted by the pleural membrane and therefore it is not made up of living cells. Usually a trauma to the chest, such as a stab wound, would allow air to enter the thoracic cavity, called pneumothorax.

The problem caused by air building up in the thoracic cavity is due to the movement of the diaphragm and intercostal muscles cannot create a low pressure relative to that of within the lungs pressure, and air rushes into the thoracic cavity instead of the lung to equalizes the pressure. Note that oxygen that enters the thoracic cavity is not usable by the individual, and only the oxygen that enters the lung can. Also, air in the thoracic cavity is not connected to the nose or mouth, or else only little air would enter the lung when aspirating.

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  • $\begingroup$ I think you got me wrong in the first part, I don't mean the fluid itself is living, I mean it was made or secreted by living cells, which are the pleural membranes cells. And since the fluid is touching them, won't oxygen diffuse into it? And sorry for the confusion, I'll edit my question in that part to make it clearer. $\endgroup$ – Manar May 6 '20 at 16:27
  • $\begingroup$ Thanks Philip, I've searched in details about pneumothorax and it seems like my knowledge lacks alot. I didn't realise the role of the pleural membranes in making the lung moves outward and the negative pressure in the pleural cavity. We tend to concentrate at school on the intercostal muscles and kind of ignore the pleural cavity that I became to think that if the external intercostal contract the volume of the lungs would automatically increase, completely missing the role of the pleural cavity. $\endgroup$ – Manar May 6 '20 at 18:27
  • $\begingroup$ @Manar lung based breathing is a really strange kluge, that most tetrapods are just stuck with, birds at least found a better solution. You should see what crocodiles have which as a weird hybrid system that has two different ways of working they can switch between. They even move the lungs around the torso to control buoyancy. $\endgroup$ – John May 7 '20 at 15:18

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