Why does lifting an IV bag increase pressure?

[Augustine Duran]

I understand that you need to increase the pressure in an IV bag to overcome the pressure in a vein so fluid pumps into it. But why does lifting the bag at some height increase the pressure?

 

[Nugatory]

What determines the pressure at the bottom of a volume of liquid?

 

[Augustine Duran]

ρgh+p_o

 

[Chestermiller]

ρgh+p_o

That would be the equation for static equilibrium. But, what if the bag were accelerating upward at acceleration a?

 

[Dale]

ρgh+p_o

Exactly. Raising the bag increases h.

 

[Chestermiller]

Exactly. Raising the bag increases h.

My understanding was that h is the depth of fluid in the bag, which does not change when the bag is raised. But, now I see what the OP meant was that h is the total elevation of the upper surface of the fluid in the bag above the outlet of the IV tubing.

 

[Augustine Duran]

im only studying static fluids right now. i guess to clarify better... why would an iv bag apply more pressure to the vein at a higher height then a lower one. I understand the math behind it, I am looking for more of a conceptual explanation.

 

[berkeman]

to overcome the pressure in a vein so fluid pumps into it

Just to clarify, we use veins for IVs because the pressure is low (the diastolic pressure) and not pulsating. If one were to try to use an artery for an IV start -- well, let's just say that you know right away if you miss the vein and hit an artery...

 

[Nugatory]

im only studying static fluids right now. i guess to clarify better... why would an iv bag apply more pressure to the vein at a higher height then a lower one. I understand the math behind it, I am looking for more of a conceptual explanation.

Imagine for a moment that you had no bag, just the tube (which in fact is close the actual situation - the primary function of the bag is as a reservoir to feed fluid into the tube). The pressure at the bottom of the tube is determined by the weight of the water in the tube above, and the higher the upper end of the tube the greater that weight.

 

[Chestermiller]

Imagine for a moment that you had no bag, just the tube (which in fact is close the actual situation - the primary function of the bag is as a reservoir to feed fluid into the tube). The pressure at the bottom of the tube is determined by the weight of the water in the tube above, and the higher the upper end of the tube the greater that weight.

I think what he's saying is, if the weight of fluid in the bag and tubing doesn't change, why does the pressure change when the bag is raised? ( I know the answer to this. I just wanted to clarify his question.)

 

[Chestermiller]

im only studying static fluids right now. i guess to clarify better... why would an iv bag apply more pressure to the vein at a higher height then a lower one. I understand the math behind it, I am looking for more of a conceptual explanation.

When the tubing is somewhat coiled (and not perfectly vertical), the walls of the tubing support part of the weight of the fluid. As the bag is raised, the walls of the tubing become more vertical, and support less of the weight of the fluid, while more of the weight is applied to cause pressure at the vein. That's why it is not the total weight of fluid in the bag and tubing which matters, but only the difference in elevation between the fluid level in the bag and the level of the bottom of the tubing.

 

[Augustine Duran]

ahh okay so its more about putting the tubing in a vertical position so none of the tubing supports the weight, not necessarily how "high" the IV bag is

 

[Chestermiller]

ahh okay so its more about putting the tubing in a vertical position so none of the tubing supports the weight, not necessarily how "high" the IV bag is

Well, you're increasing the fluid level difference also, even though the weight doesn't change. That height difference is the key.

 

[Augustine Duran]

Ok thanks all makes sense now :D