Human hair consists of a shaft, which protrudes from the skin, and the root, which resides at the base of the hair. At the bottom of the root is the hair bulb, which is soft and thickened. The bulb sits in a pit in the skin that is called the follicle. According to KidsHealth, hair growth takes place at bottom of the follicle, in a place called the papilla.
Biology of Hair Growth
An artery in the papilla supplies blood, which provides nourishment for hair growth. Hair growth occurs as new cells form in the papilla at the bottom of the root. As new cells multiply, they push previously formed cells upward through the follicle. As the cells move upward, away from the source of nourishment, they produce keratin, which hardens the hair. As this process occurs, the cells die. The keratin and dead cells form the hair shaft, which continues to grow outward from the skin as new cells form at the base.
Hair Growth Cycle
According to FamilyDoctor, the normal hair growth cycle lasts two to three years. During this cycle, the hair will continue to grow at the rate of one centimeter per month. For hair on the scalp, about 90 percent of the hair is in this growth cycle at any particular time. After the growth phase, there is a resting phase, which lasts about three or four months. After the resting phase, the hair falls out, and a new hair begins to grow.
Pubic Hair Growth
According to the AARP, there are different stages of puberty. For girls, stage two is the stage during which pubic hair begins to grow. Girls typically enter stage 2 of puberty between the ages of 8 and 14. During stage 3 of puberty for girls, which typically occurs between the ages of 9 and 15, the pubic hair gets courser and darker. For boys, pubic hair also begins to grow during stage 2, typically between the ages of 9 and 15. Pubic hair spreads and gets darker and courser during stage 3, typically between the ages of 11 and 16.
Body Hair
According to KidsHealth, hair growth on skin occurs on every part of the body except eyelids, the soles of feet, lips and palms of the hands. Hair growth occurs more rapidly during summer month than during winter months, and it grows more rapidly during daytime than nighttime.
Purpose of Hair Growth
According to KidsHealth, hair growth serves a number of purposes for the human body. Hair on the scalp helps to keep the body warm by inhibiting the loss of heat. Hair around the eyes and in the nose and ears help to filter out dust and other small particles. Eyelashes and eyebrows protect eyes from light.
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