I was in surgery earlier and remember when the nurse put injected anesthesia and something else through the IV and I could feel the liquid push itself into my vein and it felt like my vein in my hand to elbow would explode.
That got me thinking, how does my body intake so much liquid like the full IV bag without exploding or peeing on the operating table while I’m knocked out.
In: Biology
Veins aren’t rigid like home plumbing. They are somewhat elastic. Veins expand and contract for many reasons; Cary nutrients, fats, oxygen, and can expand and distribute vaccines and other liquids as necessary.
General answer. I’m at sure others can be more specific and technical.
Veins and arteries pump liquid through your veins really quickly. It is constantly moving. So, extra fluid in the veins from an IV is no different from drinking a huge amount of liquid for it to also be absorbed. Once the nutrients go where they need to go, the kidneys filter out the excess. And you would absolutely urinate on the operating table, but you most likely had a catheter in that was inserted and removed while you were knocked out.
The sensation of feeling the injection into your veins isn’t really about volume, but more that what was injected was an irritant. That’s why it is diluted with neutral IV fluids from the full bag.
Veins can expand, your kidneys can increase the rate of urine production, and yes, you would pee during surgery. That’s why they insert a catheter into you.
vessels are elastic to a point, if they shove too much in the wrong one you can cause a blowout. You’ll occasionally see people with massive “bruises” from this happening, or from the nurse putting the needle all the way through the vein. With small ones it’s not the end of the world but still definitely not ideal.
As for the “full bag” part your body absorbs the liquids as they go through your system, so it’s just a matter of not delivering liquid faster than your body can do something with it, it’s not a closed system like oil in your car, it’s constantly depositing material and picking up new things to move around.
That’s really simplified but it’s the gist.
I’ve had fluid replacement in hospitals, and first hand found out what happens to any excess.
Being severely dehydrated, I couldn’t urinate. 5 minutes after IV fluid replacement, that wasn’t a problem anymore. Kidneys work pretty fast getting rid of excess.
good answers here. also:
1. the average adult has a plasma volume of probably 3.5-4 liters. shots (i’m assuming you mean immunizations or IM antibiotics etc) are orders of magnitude smaller in volume… on the order of mL. so it’s like drops in a bucket. (and technically, those don’t usually go directly into the blood stream… they’re placed in muscle tissue where the can be absorbed and they end up in the blood stream that way).
2. sometimes fluids are put directly into the bloodstream (always veins not arteries). Almost all medications (from antibiotics to blood pressure medicine to painkillers) have IV formulations. If having direct access to the blood stream is advantageous for healthcare workers (like in hospitalized patients with multiple prescriptions, patients undergoing surgery, chemo patients and many others) an IV is put in. though they’re usually less concentrated and come diluted in saline, the answer for medications is the “drops in the bucket” answer from above.
3. even if the “drops in the bucket analogy wasn’t true, we have “volume reservoirs” in our body. the biggest one is the venous system. like people have said here, veins aren’t like rigid plumbing piping. they have highly elastic walls that have enormous potential to stretch to handle more fluid while keeping the internal pressure of the fluid inside within a tightly regulated range. thats why if you chug a liter of water in like 10 minutes your blood pressure doesn’t just spike to dangerous levels. after the water goes through your stomach to your intestines, it’s absorbed and taken into the blood stream. the veins can relax to accomodate the increased fluid. eventually the kidneys take the excess and you’ll pee it out. (the kidneys are p damn cool, they are amazing at keeping the fluid and electrolyte balance in the blood in a tightrange, that’s why you can drink a TON of water without “watering down your blood” your kidneys will just make watery pee… and if you’re dehydrated, your kidneys will hold on to as much water as possible and make really concentrated pee).
4. the situation in which a large volume of liquid IS put directly into the blood stream is if you are getting hydrating fluids like saline. in that circumstance saline IS put directly into your blood stream usually it doesn’t create an excess that needs to be handled because you’re getting the fluids you need… usually it’s run slowly into your IV and your body handles it exactly the same way it would handle you drinking that much water (it just cuts out your mouth and guts). In circumstances when you are extremely dehydrated or you have lost large amount of blood the IV is run more quickly, but again, here you’re just getting the fluids you need so there isn’t really an excess to handle.
In addition to the other answers. If the nurse needs to inject with a syringe directly into the IV connection there are guidelines for how quickly they should push in that plunger.
A syringe with just a bit of propofol-ketamine might go in rather quickly and send you to la la land. With a bigger amount of fluid they might push a bit, count to two, push a bit more, etc.
I get hydrocortisone every six weeks and while most nurses hang a tiny IV bag to drip a few have just put it in a syringe and injected it very slowly into the IV while counting with the second hand on their watch.
Ooh, just covered this in the capstone class for my major.
Veins are *significantly* more compliant (stretchy) than arteries or capillaries. At any given time, most of your blood is in your veins, and they’ll expand or contract depending on your current blood volume + osmolarity.
If you get an infusion through an IV? It’s fine. Your veins can handle a little stretch while the fluid gets into your circulatory system.
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