How do hairs on your arm know when to stop growing?
In: Biology
And how the hell does hair start growing out of places that never had hair before? Back, ears, etc.
Hair grows in three stages and cycles through these over and over. Your DNA programs your hair follicles to stay in the growth stage for a specific amount of time before resting. When the resting cycle ends it starts growing a new hair, pushing the old one out.
They don’t stop growing, they fall off when then reach a certain length and a new one replaces it. Obviously you never notice because hairs don’t fall off at the same time, it would be funny if they did though.
Simple answer: they fall out. Every hair on your body has an average lifetime that depends on where it is on your body. Hair falls out when it reaches its natural length, and a new hair starts to grow in its place. Arm hair falls out way more often than head hair. Head hair can grow for something like 7 years before it falls out. This happens all the time, so if you never got a haircut or a shave, your head hair and body hair would max out wherever the natural length is for each body part. Since we don’t want to look like cave men and women, we usually trim up the places where it tends to grow too long.
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Hair of all types is created by a follicle under the skin that grows it and pushes it upwards. Each follicle can make multiple strands, and each strand has 3 phases of life, and as it ages through these stages it grows less and less as the follicle focuses on a younger hair. Once it reaches the final phase, which for arm hair can last about 100 days, it doesn’t really grow at all, and then naturally falls out to be replaced by a new hair at phase one again. This process is repeated throughout your life, though each type of hair in each part of your body has different lengths on each of it’s phases. Human hair tends to fall out semi-randomly, which allows us to have quite a lot of variation in our hair length, unlike the fur of many other mammals which often has a very reliable life-cycle based, and is why they have totally uniform hair lengths.
Shorter hair like eyebrows and arm hair don’t stop growing. They periodically shed. The hair on your head goes a much longer interval before shedding.
I would also like to know why sometimes you get a random hair that isn’t like the others. I have semi light arm hair except 1 black hair that grows really long
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