Eli5.. What keeps the hairs on my arms and legs from growing indefinitely? Why do they never get longer than like 2 cm but grow back right after shaving?
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The length of a given hair is proportional to the length of the follicle. Your arm hairs have really short follicles, so they can only get a few cm long before they fall out.
Your arm hairs are actually falling out and regrowing all the time, you just don’t really look that closely to notice it
All the hairs on your body have a maximum length they will reach. This is because hair follicles operate in phases. In one phase, the follicle is dormant and doesn’t grow hair. In another phase, the follicle actively grows a hair. In the last phase, the hair follicle stops growing a hair, lets the hair fall out, then goes dormant again. The length a hair will grow depends on how long the “growing phase” for that follicle is. Follicles on the arms and legs have relatively short growing phases so they don’t get very long. Follicles on the top of the head have a very long (but not infinite) growing phase. This means there is a maximum length your head hair would grow to, even if if you never cut it.
We don’t really notice these phases, because all your follicles aren’t synchronized together, so there are always a mix of active growing follicles and dormant follicles. There are many medical conditions that can cause hair follicles to go dormant and stay that way (temporarily or permanently) leading to hair loss.
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