Hair consists of a protein called keratin. It grows from a hair follicle, which is rooted in the epidermis layer of the skin. The follicle is a living cell, and undergoes a cylce of growing and resting, regulated by hormones, nutrition and ageing. It also depends on where the hair is growing on the body. The base of the follicle is the hair bulb, which is where cells grow and divide to enable the hair shaft to grow.
The hair shaft has three layers: the medulla or core of the hair, which is only seen in the thick, large hairs. Next there is the cortex, which gives strength, texture and colour to the hair strand. Finally, the cuticle is thin and colourless, and is the outer layer of the hair.
Most of the hair on the body is tiny and colourless, known as vellus hairs. Only a few areas of the body have no hair at all, including the palms of the hands, soles of the feet, lips and external genitals. The thicker, pigmented hair that grows on eyebrows, eyelashes and the scalp are terminal hairs. Interestingly, these are the only hairs that are present at birth. During puberty, some vellus hairs are affected by hormones and change into terminal hairs, becoming thicker, darker and longer. The hairs affected are normally on the face, underarms and genital areas.
Hair growth rates vary from person to person, but an average growth rate is about 2.5 cm a month, and depending on where it is on the body, can last for several months or even years. Hair growth goes through a cycle. The growth stage is known as anagen. The catagen or transitional phase means that the hair growth is slowing down and hair follicles shrink. During the telegenic or resting phase, the hair growth stops and the old hair is detached from the follicle and falls out. A new hair will begin growing, which helps to push the old hair away.
How much hair do we have?
Of all mammals, humans have the least amount of hair. The hair they have still covers most of the body, but it is fine and cannot really be seen unless you are looking closely.
The average human head will have around 100,000 hair follicles and each follicle is able to grow about 20 individual hairs during the body’s lifetime. People lose about 100 strands of hair a day, on average. A human eyebrow has around 250 hairs, but if it has never been plucked, could have as many as 1,100 hairs! People also have around 150 - 200 eyelash hairs on their top lid and 75 - 100 hairs on the bottom.
The amount of hair people have can alter during their life due to hormonal changes. As women age, they can gain more facial hair around the chin and neck area, and the hair on their head can thin. Men can develop male pattern baldness, a condition where the thick, coloured hair on the top of the head is gradually replaced by shorter, thinner hairs.
Function of hair
The function of hair includes the regulation of body temperature. This is done by helping perspiration to evaporate and by trapping body heat. Hair also protects the skin, and acts as a sensory organ to help us to understand our surroundings.
Humans also use their hair for expression and for social standing within society, for exampl through hair styling. Men have the option of growing a beard or moustache alongside head hair, too. Social convention can sometimes influence humans to remove some of their body hair, for example through the shaving of underarm or leg hair.
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