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Hi!
Is it OK to use gas spaces here?

Conversely, when ascending, the surrounding pressure decreases and the sinuses and middle ear become relatively over-pressurised, forcing air to spontaneously flow out of them. If an Eustachian tube or a sinus entrance closes, or even narrows, during ascent, air becomes trapped and pressure inside these gas spaces increases.
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Conversely, when ascending, the surrounding pressure decreases, and the sinuses and middle ear become relatively over-pressurised, forcing air out of them spontaneously. If a Eustachian tube or a sinus entrance closes or even narrows during ascent, air becomes trapped, and pressure inside these spaces increases.
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Thank you, Mister Micawber.
But if you had to invent a word for these spaces, what would you use? Because there is a sentence following this passage: Pressurised air can be forced into the skull through any weak spot in the cranial structure. Once the pressure equalizes between the gas spaces and the intracranial contents, this hole will functionally close.

(bad formatting fixed by mod)
In English, we do not have to invent a word for the spaces; they are spaces. Using the word 'gas' is a misnomer, since that is not necessarily what is contained in them. I am not an anatomist, but believe these spaces would all be called 'sinuses'.
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Thank you Mister Micawber.