Hair is made from a protein known as keratin. The hair shaft is made up of three layers, known as the cuticle, the cortex and the medulla. The outer layer is the cuticle which is composed of overlapping cells which are like fish scales. The cortex comprises the hair’s main bulk and colour cells. The medulla is the inner layer and is a thin core of transparent cells and air spaces.
The average number of hairs on the human scalp is 120,000. Hairs grow from single follicles, indentations in the skin. We are born with a specific number of follicles, which we cannot change. The size of the follicle will determine the thickness of the hair. The shape of the follicle will determine how straight or wavy the hair will grow.
Hair grows about 1 cm a month. It grows faster in the summer than the winter because the sun increases mitosis (cell division) in the basal layer of the hair follicle. The growth phase anagen lasts of 2-7 years. So someone who has a 7 year growing phase may grow their hair 84cm for example. At he end of anagen the hair goes into catagen and then to telogen where it rests for about 2 months before falling out. The hair will then be replaced and the cycle will be repeated.
Each hair follicle is independent having its own growth cycle at different times; otherwise all your hair would fall at once! Instead you normally lose only 80-100 hairs a day. Of course the cycle can be disrupted by certain metabolic circumstances in the body and that is when we will have increased shedding.
Our hair has certain protective functions. Scalp hair shields us from the wind and from the sun’s burning rays. It also keeps us warm in cold weather – up to 25 per cent of body heat is lost through the head. Hair also gives some protection to some extent against knocks and bumps.
Eyebrows protect the eyes from sweat and the glare of the sun. Eyelashes provide a shield against dust and grit entering the eye. Also the fine hairs that line the nostrils act as a filter, trapping dust and other particles so they don’t enter the lungs. Similar protective hairs are also found around the openings to other body organs such as the ears.
A healthy scalp hair can stretch up to 30% its length, can absorb its weight in water and can swell up to 20% of its diameter. Its strength is greater than that of copper wire of the same diameter and an average head of hair can support 23 tons in weight.
Follicular Units with X50 magnification