Welcome to brain Stuff from how stuff works dot com where smart Happens. Hi Am Marshall Brain with today's question, 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, 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 that 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, the hair shaft breaks off, so the existing hair falls
out and a new hair takes its place. Therefore, the link 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 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. 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. Do you have
any ideas or suggestions for this podcast? If so, please send me an email at podcast at how stuff works dot com. For more on this and thousands of other topics, go to how stuff Work dot com.