Medicine & Physics -- Question about chest tube suction techniques

[Sarah00]

Homework Statement

Hi,
I'm a junior doctor. It's been so long since I had physics!
I've question about tube that's placed in chest cavity to drain air/fluid/etc.

The second bottle, is under-water seal bottle.
I know that its purpose is to prevent air moving TO the patient. but how?
The "trap bottle" has air! why can't it go to the patient??

Homework Equations

P=F/a?

The Attempt at a Solution

 

[Sarah00]

I don't think my question was very clear..

The picture was about chest tube.
Which has different types.
The pic in previous post is called 3-chamber..

there is another thing which is single-chamber:
which is in figure A:

This is pretty easy .. like if I have a cup filled with water and a straw. I can't suck air from the cup but I can blow air into the cup
but in 3-chamber. why I can't suck air from the bottle?

 

[mfb]

Trying to do so would raise water in the pipe of the middle bottle, until the pressure difference stops the reverse flow (or until liquid from the middle bottle reaches the left bottle, or you drain so much liquid from the second bottle that air does enter the pipe there).
Ideally, this stops reverse flow before air reached the end of the pipe going to (coming from) the patient.

 

[Sarah00]

Thanks.
"until the pressure difference stops the reverse flow" can you explain how? by reminding me with the physics principle or something like that

 

[mfb]

Sucking up water in the middle bottle needs a lower pressure in the left bottle (compared to the middle one). A higher water level in the pipe needs a larger pressure difference. For every fixed pressure difference there will be some equilibrium, as long as the water does not rise high enough to flow downwards into the left bottle.