This question involves several parts:
The outside atmosphere, the liquid inside the bottle, the flow rate observation window, the needle, and the blood in the human vein.
The relationship between their pressures is:
The outside atmospheric pressure ? is slightly less than, and essentially equal to? The pressure of the medicine inside the bottle;
Because: the liquid is constantly flowing out, the pressure inside will drop, but a slight drop will be sucked into the air, always in dynamic equilibrium.
The pressure of the liquid inside the bottle? is less than ? The pressure of the liquid (and also the gas) inside the flow rate observation window?
Because: this is obvious, the pressure is greater down there.
Pressure in the flow rate viewing window? is less than ? Pressure of the liquid in the needle
Because: this is obvious, the pressure down there is big ah.
The pressure of the liquid inside the needle is greater than? Human venous blood pressure
Because: this is obvious, otherwise the blood would flow back.
The human venous blood pressure? is greater than? Atmospheric pressure
Because: this is obvious, insert an empty needle and blood will flow out.
That is to say, if the IV bottle is lowered the blood will flow outward, and the medicine will not be able to be pumped in,
but by raising the height of the bottle, a pressure difference is created, and the medicine is successfully pumped into the veins of the human body.
Answering the question:
Why wouldn't that column of air enter the bloodstream with the liquid?
You should be able to notice that there is always half a tube of liquid "in the flow rate observation window",
it's right there, it's not increasing or decreasing,
because the liquid is always underneath under the force of gravity, blocking the exit, and the air is always on top and can't get to the exit;
And the liquid is constantly replenished while it's flowing downwards. There is a constant replenishment of the liquid as it goes down, and the level doesn't go down,
so the column of air doesn't enter the bloodstream with the liquid.
But this is not absolute, provided that the "flow rate viewing window" is placed vertically, which would be a tragedy if you turned it upside down.
If you look more closely, or ask a nurse,
This phenomenon is utilized in the preparation of the drip.
Why wouldn't that column of air rise up that? Bottle of medicine? in it?
The air would have had a chance to get up there, but the tubing was too thin,
and the liquid surface film formed as the medicine flowed down prevented the air from rising into the bottle.
Think about it, the liquid goes down and the air goes up, it needs to go one way or the other,
that tube is too thin and there is no way to form a channel for the air.
What if the small cylinder? Connect with the outside world. Then the gas inside the small cylinder would not be ? pushed out? which in turn causes the tube to leak?
Answer: exactly!