How does hair length work?

As hair cells form in follicles, they push other cells out of the follicle. Check out this HowStuffWorks podcast to learn more about your hair's growth and rest phases.

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Episode Transcript

Speaker 1 (00:00):
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Welcome to brain Stuff from how Stuff works dot com
where smart happens. Hi. I'm Marshall Brain with today's question.

(00:30):
Why does the hair on your arms stay short while
the hair on your head can grow very long? Each
hair on your body grows from its own individual hair follicle.
Inside the follicle, new hair cells form at the root
of the hair shaft. As the cells form, they push
older cells out of the follicle. As they're pushed out,

(00:52):
the cells die and they become the hair that we see.
A follicle will produce new cells for a certain period
of time, depending on where it's located on your body.
This period is called the growth phase for the hair.
Then it will stop growing for a period of time
This is called the rest phase, and then start the
growth phase again. When a hair follicle enters the rest phase,

(01:14):
the hair shaft breaks off, so the existing hair falls
out and a new hair takes its place. Therefore, the
length of time that a hair is able to spend
growing during the growth phase controls the maximum length of
that hair. The cells that make the hairs on your
arms are programmed to stop growing every couple of months,
so the hair on your arms stays short. The hair

(01:36):
follicles on your head, on the other hand, are programmed
to let the hair grow for years at a time,
so the hair can grow very long. Animals that shed
have hair follicles that synchronize their rest phase, so all
the follicles enter the rest phase at once. This way,
all the hair falls out at the same time. A
dog that sheds will lose its hair in large clumps.

(01:59):
Some animals can also switch the coloring agent in the
hair follicle audit off, so in the summer of the
hair is pigmented brown with melanin, but in the winter
it's not pigmented, leaving the hair white. For more on
this and thousands of other topics, because it how staff
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