How do local anaesthetics stay local for several hours instead of transferring around your body through the bloodstream?
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Short answer: they don’t. The medication is constantly being taken into the blood stream by the capillaries. There is a list of locations throughout your body where it is absorbed into the bloodstream fastest that you have to memorize in anesthesia school to avoid local anesthetic toxicity.
However in the dental locations there is a combo of lots of extra medication being injected (it only takes a drop to cause the nerve to stop conducting, but that would only last a few minutes before being cleared, so they usually inject several cc’s) and the injection site having slow clearance of the local (as opposed to next to a major nerve bundle in the shoulder or upper leg, which is next to major arteries and nerves).
The other half of the equation, is that the local anesthetic penetrates the layers of the nerve, so even when the anesthetic is cleared away from around the nerve, it still has to be cleared from the nerve itself which can take awhile.
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