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Lungs: Intrapleural fluid, breathing mechanism

Hi,

I'm struggling to piece together the information I have on intrapleural fluid's role in the mechanism of breathing. My understand is that:

- Inspiration: Contraction of the diaphragm and intercostal muscles increases the volume of the thoracic cavity which basically brings the chest wall away from the lungs which increases the volume of the intrapleural cavity thereby decreasing its volume (Boyle's law). Therefore the pressure in the intrapleural cavity is less than the lungs which drives air into the intrapleural cavity (??) and that equilibriates lungs and intrapleural cavity's pressures but means that the lungs P is less than atmospheric was causes air to enter the lungs?

However, this seems to be simplified as it suggests that the volume of the lungs is constant? My notes say that there is surface tension on the alveoli pulling them inward (does this actually occur or does surfactant stop this completely? Or does surfactant just reduce it? Also is it correct to say that surfactant reduces the hyrophilic attraction between water molecules and that's how it works?, there is also elastic recoil of the lungs pulling them inward as well as the recoil of the chest wall which is apparently outward??

Is it correct to say that during inspiration the net result is the increased intrapleural fluid cavity volume?

I also don't understand is air actually diffusing into the intrapleural fluid?? Why is it going in there instead of capillaries?

Sorry for all the questions. :colondollar:

Reply 1

nexttime

Wow a lot of confusion here.

Original post by LeaX

- Inspiration: Contraction of the diaphragm and intercostal muscles increases the volume of the thoracic cavity which basically brings the chest wall away from the lungs

No! It moves the chest wall outwards and the lungs move with it. Therefore the lungs get bigger.

which increases the volume of the intrapleural cavity thereby decreasing its volume (Boyle's law).

The increased volume results in decreased pressure.

Therefore the pressure in the intrapleural cavity is less than the lungs which drives air into the intrapleural cavity (??)

No! It would drive air into the lungs.

Air in the intrapleural cavity would be a pneumothorax, a consequence of damage to the lung letting air escape. This would not normally happen. The pleural cavity is normally lined by only a thin layer of fluid (which causes the adherence of the lungs to it) and no air at all.

However, this seems to be simplified as it suggests that the volume of the lungs is constant?

Its definitely not.

My notes say that there is surface tension on the alveoli pulling them inward

Yes. Elastic recoil of the chest is inwards. This means that all of the work of breathing is done when sucking air in. No energy is needed to expire (though if you want to force expiration the main muscle that does so is the rectus abdominis i.e. you abs). You can observe this when someone is on a ventilator for instance - a ventilator only pushes air in, it does not need to suck it out.

(does this actually occur or does surfactant stop this completely? Or does surfactant just reduce it?

It just reduces it, but significantly so. Babies born too early need surfactant urgently to preserve lung function and survive.

Also is it correct to say that surfactant reduces the hyrophilic attraction between water molecules and that's how it works?,

Water generates surface tension due to its polar nature and the movement of molecules on the surface. Surfactant is a large molecule that interrupts and reduces this. Ask a chemist if you want more detail!

there is also elastic recoil of the lungs pulling them inward as well as the recoil of the chest wall which is apparently outward??

Maybe, but the net effect is inwards.

Is it correct to say that during inspiration the net result is the increased intrapleural fluid cavity volume?

Definitely not

I also don't understand is air actually diffusing into the intrapleural fluid?? Why is it going in there instead of capillaries?

It isn't except in a pathological state.

Reply 2

blossomx

Original post by nexttime

Thank you so much. I'm still struggling to understand why the transpulmonary pressure affects breathing though? I understand that a drop in alveoli pressure brings air into the lungs and vice versa but I don't understand the relevance of the intrapleural pressure?

Reply 3

nexttime

Original post by LeaX

The intrapleural space is completely collapsed under normal conditions. This is because it has 'negative pressure' i.e. the lungs are higher in pressure and expand into it and effectively stick to the inside of the chest wall. It doesn't affect breathing beyond this under normal conditions, air does not go into the pleural space and transpulmonary pressures are not a significant consideration when breathing.