chest tubes

I found that it helps keep negative pressure in intra pleural space and allows fluid to exit. I guess intra pleural/intra thoracic is confusing me.

so it restores negative pressure but it also helps fluid exit

I found that it helps keep negative pressure in intra pleural space and allows fluid to exit. I guess intra pleural/intra thoracic is confusing me.

so it restores negative pressure but it also helps fluid exit

You should think of these tubes as "pleural tubes", because "chest tubes" can mean different things.

So, the answer is in the term "water seal". There is a column of water between the collection chamber (where air and fluid drain) and the pleural space, separated by all of the tubing.

The pleural space should be filled with lung, not air or fluid.

The hope there, being that the hole in the lung will seal off as the visceral and parietal pleura creates a "scar".

The water acts as a one way valve, such that, when the patient exhales, air that leaks out of the lung ( through a hole that shouldn't be there) isn't sucked back in through the pleural tube so that eventually, any air in the pleural space (where lung should be) is, theoretically and ideally, marched out with each breath and contained "down stream" of the column of water so that the lung can re-inflate and re-occupy the pleural space.

So it helps drainage since negative pressure is in intrapleural space.