eli5 Why are air bubbles in syringes so dangerous?
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They aren’t actually. It takes a pretty large amount of air to cause an embolism. If you look at an IV that is in use, there are a lot of small air bubbles in it.
The heart pushes blood down arteries and the force of this pushes the blood ahead of it back around to the heart. Blood and fluids in general do not compress very well so pushing on fluid at one end of a tube full of it will push it out the other side, even if the tube splits into multiple directions.
However if there is a gas such as air in the tube then it doesn’t necessarily work that way. The gas can compress and expand, absorbing the force of the pushed fluid without causing the other end to flow. A significant air bubble in a blood vessel can stop blood from flowing and starve an area of tissue from the oxygen and nutrients it needs, causing it to die.
The concern is that you will receive an air embolism. An air bubble blocks blood flow past it. The cells past it starve and die. In some locations, that can be critical. If the bubble is big enough, then it can go into your heart and impede your heart’s ability to circulate blood whatsoever. That being said, not just ‘any amount’ of air is necessarily lethal, and caution is taken to remove air from syringes (in part because having a big air bubble in your syringe means your measurement of dosage is off, and you don’t want to inject “eh close enough” dosages).
Courtesy of u/stuthulhu from [this thread](https://www.reddit.com/r/explainlikeimfive/comments/44ya5s/eli5_why_do_people_say_a_syringe_full_of_air_will/?utm_source=share&utm_medium=web2x&context=3)
The short answer is that they can act like a blood clot. Anything that can prevent blood from flowing prevents oxygenated blood from reaching tissue, which causes tissue death. If that happens to occur in the heart/lungs/brain, it can cause death.
The longer answer is that in most cases, they probably aren’t that big a deal. The other people here saying they aren’t dangerous are wrong. Any visible amount of air is potentially dangerous, since it only takes nanometers to obstruct capillaries. However, the risk of that happening is very low. That risk rises based on how much air enters the system. Most tiny bubbles (especially if they enter while laying down) will get broken into smaller bubbles and pushed into the right side of the heart, and then into the lung vessels where they can be exchanged and absorbed easily. That’s the vast majority of cases. A smaller percentage of people have a persistent hole between the two sides of their hearts (ASD/PFO in atria, or VSD in ventricles) that can allow passage between the two sides without going to the lungs. That means air can skip the place that’s easiest to get rid of it, skip to the exit of the heart, and slide right on up to the brain.
The other thing that makes circulatory air problematic is that it’s very difficult to get rid of, once it’s in there. It will get slowly absorbed, but slowly isn’t really a good option when it’s blocking off blood flow to heart/lungs/brain. The goal if you get one is just to keep it from blocking anything important until you can absorb it again.
This is a much longer explanation that goes into much more detail: https://journals.sagepub.com/doi/full/10.1177/0267659117706834
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