Why Saline?

This question has nagged at me for a while: when IV fluids are administered, why is the fluid typically saline? Is there some particular reason that plain, sterile water couldn’t be used? Does a body that needs the additional fluids provided by the IV also need additional salt?

They need to use a liquid that matches your normal level of salinity, otherwise the liquid will dilute your salt level, which is unhealthy. By using saline, the body doesn’t have to do anything with the water and it works as an inert carrier.

The concentration of NaCl in the saline used (0.9%, IIRC) is approximately that of the concentration of sodium inside the cells. If you were to pump plain water into the bloodstream, all of the ions (sodium, potassium, chloride, calcium, etc) inside your cells would diffuse out into the solution to try and reach some sort of equilibrium. This would be a Very Bad Thing for your cells, since they are very carefully maintained at these concentrations by a lot of ion pumps and other mechanisms in order for them to do what they do. By using a solution that is essentially the same as that inside your cells, you don’t disrupt the ionic balance of your cells, which is a Good Thing, because it is a big part of what’s keeping you alive.

Along these lines why do they always inject (or use an IV) and put in lidocaine. I know it’s a topical anesthetic, but what does it do in your blood stream. (Not having any medical backround, I know this only from when I used to watch ER)

Exactly backwards. Ions do not “diffuse” out of cells. If cells are bathed in water, the water will osmose into the cells, and quite often the cells will be ruptured by the increased pressure.

And who the hell “always” injects lidocaine? When?

Oh, you’re right, Nametag - osmosis would be the greater effect! Silly mental slip there! Its the same idea, in that the water would try to dilute the ions in your cells, in order to reach some sort of equilibrium…

A doctor once told me that the single most painful thing you could inject into your body would be pure water.

Could this be true? (Worse than, say, battery acid?) Or, leaving aside hyperbole, would an injection of pure water actually be agonizing?


From here

Action/Kinetics: Shortens the refractory period and suppresses the automaticity of ectopic foci without affecting conduction of impulses through cardiac tissue.

Uses: IV: Treatment of acute ventricular arrhythmias such as those following MIs or occurring during surgery. The drug is ineffective against atrial arrhythmias.

I was an EMT so I am fuzzy on the pharmacology but as I understood it it basicly makes you heart less likely to respond to small erratic electrical activity.

Another question. When I rinse off my contact lenses before putting them into my eyes, I use saline. (I use a real contact lens solution in which to store them at night, so no worries there.) My eye doctor just suggested I use saline before putting them in, instead of the contact lens cleaner, because in some people, this hurts/stings their eyes. Any reason why I use saline, as opposed to plain water?

Yes, it’s the exact same reason as above.


Maybe your doctor was worried you’d become an IV drug user one fine day. It’s not unheard of for drug addicts to replace each other’s drugs with syringes of plain water. It seems that, if you are high enough, the placebo effect makes coke and speed and plain water not all that different. Healthy, healthy lifestyle, that IV drug use.

I believe this type of thing is the reason you put salt in the water while you’re cooking - it’ll prevent too much water being absorbed through osmosis, making stuff mushy.

You also use saline for nose irrigation for the same reason. Squirting pure water into the nose is painful.

You’re right about it making the heart less likely to respond to ectopic activity. Lidocaine is a sodium-channel blocker. As a result, it raises the stimulation threshold, so more electric activity is needed to stimulate a contraction.

I suspect it works in a similar manner as a topical anaesthetic, by blocking conduction in nerve pathways. Just a WAG though.

St. Urho

Info from: here