When the IV drip for a patient is completed, the patient's blood will flow back.

This is apparently due to the pressure difference.

However, IV needles are inserted into veins in the direction of blood flow.

Hence, the blood is flowing against the direction of blood flow.

Is the pressure difference so high?

Can someone explain this? A non mathematical approach is fine

  • $\begingroup$ Homework problem: given a venous pressure of $\sim$5 mmHg, how high will blood backflow into an empty IV line? $\endgroup$
    – rob
    Commented May 28, 2023 at 12:21
  • $\begingroup$ @rob is that a direct calculation from the relative densities of Hg and blood? $\endgroup$
    – Cong Chen
    Commented May 28, 2023 at 21:40

2 Answers 2


I'm not a nurse, but I think that the pressure into the vein is enough, if you see the blood flowing from the vein to the outside.

This should mean that, locally, close to the needle, a small amount of the flow changes its direction to enter the needle, while the most of it keeps flowing in the "expected" direction.

It's like you drinking with a straw from a river. No matter the direction of the straw, you can drink through it, but you're not reversing the whole flow of the river.

enter image description here

If I'd try a qualitative sketch of the pressure field, I'd say something like this, for the iso-contour of pressure.

enter image description here

  • $\begingroup$ It also really depends on the height of the IV-Bag, i.e. the pressure. During a recent procedure the nurse checked the backflow of my port to see that the flow was working as intended by dropping the IV-bag quite low, resulting in a "negative pressure" or basically a siphon, and blood flowing back in the IV-line. So direction of blood is not of importance. Your straw comparison is on point, just that its easier to drink if your straw is lower than the river. :) $\endgroup$ Commented May 29, 2023 at 13:45
  • $\begingroup$ it really depends if blood leaving the vein from the needle is somehow collected, to establish an equilibrium condition between gravity and pressure gradient (Stevino's law for equilibrium: $-\mathbf{\nabla} P = \rho \mathbf{g}$. If blood is not collected in a bag, or even in the "pipe" (I don't know the best word for it) reaching a sufficient height to establish the equilibrium, blood will keep flowing out from the needle in the "reverse direction". Maybe some biology plays a role after a period of time, but if I should know fluid dyn quite well, I don't know almost anything about medicine $\endgroup$
    – basics
    Commented May 29, 2023 at 14:03

IV bags use the age old technology of this thing called gravity. Yes some have computerized regulators on them but for the most part, any liquid in existence will always try to navigate to its lowest possible location. Also, the iv bags hold a certain amount of kinetic energy in their elastic like material that is will convert fully to static energy when completely compressed flat (like it was before it was filled). The flat bag with 0 kinetic energy is still a hooked up to the enclosed vascular system which is pressurized. Since there’s no fluid left for gravity to pull into your body, the amount of pressure in your vein becomes higher than the amount of pressure that the empty bag has keeping your blood in your arm, and blood flows backwards just for a second until the pressure equalizes. If you lower the height of that bag, decrease the the pressure thats preventing more blood back flow, you’ll see slightly more blood appear in the line.

I don’t know if any of that is true, but i feel like it’s fairly close to accurate. (This is my first post on here, please don’t hurt me. 😝)

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