What is the principle of dropper when making infusion bottles? Where does the bubble come from?

I don't know if you have noticed, but only glass bottles have bubbles.

When the infusion bottle is made, the liquid medicine is pumped into your body, and the whole device is sealed, so at this time, there is no air in the bottle, which means that the air pressure is less than atmospheric pressure, so you can only inhale from the other end, and the blood will flow back. The pipe connected with the outside can balance the internal and external air pressure, that is, the total air bubbles. Plastic pipes are not needed because of their strong plasticity and deformation at any time.

This is just a personal account, and it can't be a short answer.