eli5 Why are air bubbles in syringes so dangerous?
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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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