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The reason armpit hair doesn’t grow down to your knees

Li longs to learn: How does hair know when to grow? When you shave your legs, it grows back but it stops growing after a certain length. If you shave it again, it will grow back to that length. What’s up with that???

The Left Brain leads her:

Li, actually your question is easy to answer once you understand two things:

3 stages of hair growth

The first thing to know is that hair goes through 3 different stages as it grows: Anagen, Catagen and Telogen phases. The Anagen stage (that’s Anagen, not Anakin!) is the stage where the hair grows like crazy. This stage can last a up to 4 to 6 years and can produce scalp hairs that grow to be almost 3 feet in length! (that’s 100 cm for our international readers). And if you think 3 feet is impressive, you ain’t seen nothin’! Human scalp hair longer than 5 feet has been reported! Yikes! We’d hate to see the bill from her stylist!

The Catagen stage follows the Anagen stage. This is basically a transitional stage which means the follicle is slowing down production of the hair, not much happens here.

The third stage is the Telogen, or resting, stage. The hair stops growing and just sits there in the follicle. When the cycle starts all over again with Anagen phase, the old hair is pushed out by the new hair. That’s one of the reasons you normally shed about 100 or so hairs each day – the old ones are getting replaced by the new ones.

Two types of hairs

Ok, second thing to understand is that there are two different types of hairs: Terminal and Vellus. Terminal are long hairs (the 3 footers we mentioned) and are thicker and have a longer growing cycle (growing season like flowers) 6 to 8 years. Meaning most of the time they are in Anagen phase. These are found on the scalp, mostly. Terminal hairs are the kind you have to cut because they get too long.

Vellus on the other hand are short hairs (a millimeter or less) they are very fine, and they have a very short life cycle, which means they spend most of the time in the Telogen phase. That also means they’ll never grow as long as scalp hair. These very fine hairs are found on “hairless” parts of the body like arms and legs. (Ok, those areas aren’t hairless, but they kind of look hairless because the hairs are so tiny and fine.)

So, to answer your question, that’s how hair knows when to grow – it’s determined by the type of hair and the stage of growth it’s in. Which of course is determined by hormones. Isn’t everything?

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