IV's, Why Do They Work??

[FL Knifemaker]

When blood is taken for testing, it GUSHES out of your arm, into the test tubes.........

Yet, when you're hooked up to an IV it flows the other direction What's UP with dat???

[James Hart]

gravity?
one way valves?
not sure... good question

(I'm off to google it)

[RED5]

Originally Posted by FL Knifemaker

When blood is taken for testing, it GUSHES out of your arm, into the test tubes.........

Yet, when you're hooked up to an IV it flows the other direction What's UP with dat???

I'm gonna go with Capillary action for 10 Art...........

[Bassic83]

The blood collection vials are in vacuum (really). So the blood isn't shooting into the vial as much as being sucked into it. If you nick yourself shaving, it doesn't shoot out, it kind of leaks out. (Unless you're shaving with a straight razor, and accidentally nick your carotid artery! )

Also, the pressure from gravity makes the IV fluid have a flow pressure. Not much, but enough to overcome the blood leaking out the wound.

[Big Joe]

The tubes that take your blood have less pressure than your veins and easily take the blood in.

IV's have the pressure of the fluid weight and gravity behind them.

If you have a semi-permanent IV placement in and take the cap off it it will shoot blood at you if it in the right spot. Also sometimes IV's "infilltrate" where blood goes beyond them and into the tubing.

[FL Knifemaker]

Originally Posted by Bassic83

The blood collection vials are in vacuum (really).

OK, that explains it. I didn't know that. Vampire Vials

[Bassic83]

Ed Zacklee!
Wamphyri tubes!

[Jared Lash]

The pressure of a static fluid is equal to its density x acceleration due to gravity x its height (or depth) in addition to atmospheric pressure assuming it is open to the environment.

This is why an IV must always be elevated above a patient (also obviously for gravity to allow it to flow downward) - if it is elevated high enough (a few feet is generally enough) then it will have significant pressure to flow into the person's bloodstream instead of vice versa.

[5andFretless]

Your blood pressure is 120/80 (example) That is mm Hg. The 120 is the pressure at heart contraction and 80 is the pressure at heart rest - the pressure at the IV connection point.

1 mm Hg = about 1/2 an inch of water, so to over come the pressure of 80 mm Hg the bag of IV solution would need to be about 40 inches above the body. But that is not actually true, because there is a loss in pressure as you get further from the heart, the person is normally resting so the pressure is lower, etc.

Next time you have an IV in take the bag down and lower it. Watch what happens. The blood will start to climb up the tube at some point. It will also annoy the nurse, so be careful.

In trauma situation we used to have the firemen set/squeeze the IV bags to allow them to lay on the trauma stretcher and still get max fluid infusion.

As for the vacuum tubes, they are not that way to "suck" in the blood so much as to prevent a pressure lock in the tube when the blood flows in, and to prevent oxidation of the blood. I have seen people that can barely fill a purple top because their Diastyolic pressure is so low.

[Bard2dbone]

Okay. these answers are pretty close. But I've done thousands of IV's so I'll weigh in with the other parts.

It's all about pressure differential. They were right when they said blood 'gushes' into the lab blood tubes because they are at a very low pressure, also the blood in that vein is at an artificially elevated pressure because of the tourniquet that the phlebotomist put on you (that rubber band thing around your upper arm) It's not an extremely effective tourniquet - blood still gets past it to supply your arm. But the blood returning up your arm toward your heart is slowed. That's why the vein stands up real big. It's acting like a little vein-shaped water balloon.

Take the tourniquet off and the pressure in the vein decreases, allowing the blood to flow normally and the vein to shrink back to normal size.

Now here's where it gets a bit odd. The IV solution runs freely into your vein partially because of the weight of the fluid, and partially because it is actually sucked into the vein (well okay just a TINY little bit, but some) through a process called eduction. Your blood actually (sort of) PULLS the fluid in.

If the IV infiltrates, that means it isn't flowing where it is supposed to go, but is still flowing somewhat. This is when you sudden;y look like you're wearing a catchers mitt and your whole arm aches for a couple of days. SOme of the fluid may be going into the vein. But the important part for the sake of this example is the fluid just going into the space around the vein and pooling. It's not fun.

[spade2you]

These days most IVs are run in through some sort of pump. They're evelvated, but don't really need to be as there are calibrated IV pumps that usually infuse the IV fluids at a predetermined rate.

[TrooperFarva]

Originally Posted by Bassic83

The blood collection vials are in vacuum (really). So the blood isn't shooting into the vial as much as being sucked into it. If you nick yourself shaving, it doesn't shoot out, it kind of leaks out. (Unless you're shaving with a straight razor, and accidentally nick your carotid artery! )

Also, the pressure from gravity makes the IV fluid have a flow pressure. Not much, but enough to overcome the blood leaking out the wound.

+1