Why doesn't the air pipe of the medical infusion set flow out of the liquid, but does the vertical d

1、 The evolution of intravenous infusion

Intravenous infusion system has developed to the third generation. The upper part of the first generation liquid container is completely open, which is in direct contact with the air, which has a greater safety risk. In the second generation, the upper end of the liquid container is closed, and the air pressure inside is balanced through the intake pipe. The third generation was completely closed, but the liquid container was changed to soft bag. (refer to figure 1)

2、 Principle overview

In order to achieve the purpose of medicine flowing downward, the conditions must be satisfied at T1: the downward force of liquid must be greater than the upward force, that is (top gas pressure P1 + upper hydraulic P2) > venous pressure P3.

In the first generation of infusion system, P1 is equal to atmospheric pressure; in the second generation of infusion system, P1 is close to atmospheric pressure through the air inlet pipe; in the third generation infusion system, although there is no air on the upper part of the container, the liquid in the container is subjected to 1 atmospheric pressure through the bag wall because the liquid container is changed into a soft bag. Because P1 is relatively fixed, we can change P2, that is, adjust the height of the container, to achieve the purpose of medicine flowing down.

The question is, why is the air flowing through the inlet pipe while the other one is the liquid medicine? How does the air from the intake pipe enter the container?

3、 About the intake pipe

（1） Intake pipe structure

In fact, the end of the intake pipe has a very delicate design. The middle sheet allows the air to pass freely, but it will block the outlet under the liquid pressure from the inside to the outside. Its function is to allow the air to enter and exit freely, but it can prevent the liquid from passing through (except when the pressure is too high). It is such a design that ensures the perfect operation of the second generation infusion system. (refer to figure 3)

（2） Physical process

When the second generation infusion system is working, the intake pipe actually changes three states in turn: full of liquid medicine, liquid medicine reflux and air circulation.

1. Filling stage

When the tip of the infusion set is inserted into the container, under the action of P1 and P2, the liquid will overcome the resistance below and flow down the inlet pipe and the infusion pipe respectively. Since the switch of infusion set is usually closed before injection, the liquid will enter part of the infusion tube and fill the air inlet pipe.

2. Liquid reflux stage

At T2, the solution was subjected to inward external air pressure P0 and outward P1 + P4. When the infusion switch is turned on, as the drug solution overcomes the venous pressure and enters the patient's body, P1 gradually decreases due to the increasing volume of gas in the upper container. When P1 + P4 < P0, the medicine liquid will gradually return to the container. In theory, the speed of reflux should be equal to that of the medicine flowing down the infusion tube.

3. Air circulation stage

When the liquid medicine in the intake pipe flows back to T3, the external air further moves towards the inside of the container under the effect of pressure difference. At this time, the air encounters a new outward resistance, namely the surface tension of the liquid. As the air moves inward, the liquid surface is pulled inward into a semicircle or an ellipse, until it completely overcomes the surface tension and becomes a closed bubble. Due to the liquid height difference, the upper and lower surfaces of the bubble are subjected to two forces with different sizes and directions, and finally an upward resultant force is generated to guide the bubble to move up to the top air area, and P1 will increase accordingly to complete a cycle.