Hair and fur can be found only on mammals, and all mammals have at least some hair or fur on their bodies at some time in their lives. On this page we’re going to look closely at human and mammal hair.
The outer layer of your skin is called the epidermis, and it’s full of holes. There are pores, sweat glands, and hair follicles, which are the pits from which hairs grow. Follicles extend well down into the dermis, or ‘underneath skin’, and grow from a bulb, or root.
Hair follicles are often next to a sebaceous gland, which releases oil to lubricate and protect the hair shaft. The follicle holding the hair shaft is also connected to a small muscle, called an arrector pili muscle, which can cause the hair to stand erect, and which also pulls down the skin into a little pocket. This is what happens when you get ‘goosebumps’. Animals can control this muscle … a dog can make its hairs bristle out, and a porcupine can raise its quills … but humans can’t. It’s an involuntary muscle.
A hair shaft is made from a protein called keratin (the same one your fingernails are made from), and has three layers. The outer skin of a hair, called the cuticle, is made from dead cells that form scales, and give the shaft strength. Inside the hair is a layer called the cortex, which provides the colour, and inside that is the the medulla.
At the bottom, under the skin inside the follicle, the hair root produces the cells that form the living part of the hair. This all happens inside the bulb. This new growth pushes the cells that already exist up and out from the follicle. As the hair is pushed upwards through the skin, the hair loses its DNA and becomes just a strand of proteins. This protein strand (the hair you comb or brush) is not living tissue. The only living parts of hair are the cells within the hair follicle, under the skin.
The colour of hair is determined, just like your skin, by how much melanin is in the cortex of the hair. Melanin is a protein, and there are several kinds; one of them is a pigment (it has a dark colour). Black hair has lots of this pigment; white hair has none.
Animals actually have several kinds of hair, or fur. Guard hairs lay over the inner fur to protect it. Sometimes these guard hairs can be very unusual. For example, the guard hairs on a porcupine are its spines. On a lion, some of the guard hairs are long and thick, on its mane. The guard hairs of a polar bear are translucent, allowing light to penetrate to the skin.
Underneath the guard hairs is the fur, which can be three kinds … underfur (which is always growing), regular fur that is short, and velli (down, or fuzz).
Animals also have hairs called vibrissae, or whiskers. These are straight and stiff, and are connected to nerves below the skin which are very sensitive. They provide the animal with information about its immediate surroundings.
Most hair, on humans and animals, is continually being shed, or lost, as it falls out. This is called molting, and humans do it too! An animal may shed all its hair in just a few weeks, which will be replaced by hair of a different colour or texture, perhaps because it is becoming an adult, or perhaps in preparation for a change in the weather. They do this only in certain seasons.
In humans, however, hairs fall out only a few at a time, and all year long. Hairs are constantly molting and being replaced.
What is hair for? Here are some reasons why animals have thick, coloured hair:
- insulation to keep heat in during cold weather
- camouflage, helping the animal hide from predators
- signals to other animals (eg: the deer’s white tail)
- protection from injury
- warnings to other animals that it is dangerous (eg: the skunk’s colours)
Human hair no longer serves these purposes, of course, since we’ve been using technology (even if just a club and fire) for a few million years, and our survival has become less dependent on body hair; we’ve evolved into people who are much less hairy than we used to be.
The hair we do have comes in many different forms: scalp hair, facial hair, body hair, armpit hair, and pubic hair. But mostly human skin is now relatively hair-free, compared to animals. Various types of body hair are different in structure and chemical make-up. Hair also varies in shape, length, and rate of growth. For example, scalp hair or male facial hair is not at all like the fine, downy hair found on the face of infants or females.
Sunlight, hormones, temperature, and nutrition all affect the rate of hair growth and its strength, as does body location … for example, head hair grows fastest, averaging about a third of a millimetre per day, or a centimetre per month.
Let’s look at some hair close up …
The cuticle, or outer layer, of a healthy shaft of hair. The scales lie flat, and are covered with oil, making the hair feel smooth and look glossy. The scales make the hair stiff, and protect the inside.
A hair with a damaged cuticle, perhaps from brushing, colouring, or blow-drying. The hair looks dull, and scales flaking off may expose the inner layers, leading to further damage.
A hair which has been broken, possibly by a hair elastic twisted too tightly. Conditioning agents can improve the look and manageability of damaged hair, but can’t repair the shaft.
A split end, caused when a hair shaft is pulled apart. Many split ends give hair a frizzy appearance, and cause tangles.
Hairs are different shapes. If you look at the end of one that has been cut, the end will either be round or flattened. Hair shape can vary both with its location on the body, and with ethnic type. Some hair has an oval cross-section, giving it a slight wavy curl. Hair can also have a flattened oval cross-sectional shape, making the shafts very curly or kinked. Other hair is circular, or round, in cross-section, making it very straight.
Hair is also very elastic. When it is wet, it will stretch along its length by as much as 30% without damage. It also absorbs water, causing it to swell. However, both elasticity and moisture content is lower in black curly hair than in lighter colours.
|Hair can be reshaped by styling, using a strong chemical, which will break the chemical bonds that keep the wall of the hair shaft stiff. Shaping can be done when this happens, giving the hair a curl, or straightening it, as desired. Afterwards, another chemical is added to neutralize the original one, and to allow the bonds to form again in a new configuration. This is called a ‘permanent’ because it permanently alters the shape of the hair shafts; the original shape returns only after the shaft has grown out far enough. This kind of treatment, especially if applied frequently, weakens the hair shafts.|
Some of hair’s stiffness also comes from weaker chemical bonds which form between the molecules of keratin on the outer surface scales. Breaking these bonds will also allow you to change the hair’s curl. This can be done simply by wetting the hair, and then fastening it into the desired shape as it dries. Wetting the hair again, however, even through the humidity in the air, will allow the hair to return to its original shape as the keratin molecular bonds rejoin.
Hair does not grow all the time. Of the 100,000 to 150,000 hairs on an average human head, 90% of them at any one time are growing. Hairs may stop growing for a while, and then begin again. Sometimes when a hair stops growing, part of the root dies, and the shaft breaks off. The hair may fall out, or be pushed out by the next hair growing from the follicle. Normally, up to 100 head hairs are shed each day. So some of that shed hair in your brush is quite normal.
Head hair will last a long time before it naturally dies and falls out … sometimes up to 5 years. Eyebrow hairs, on the other hand, last only about 5 months before they are shed.
When newly growing hair on the head doesn’t appear as fast as hair is shed, the number of hairs on the head starts to decrease, eventually leading to hairless (bald) areas.
Sometimes we want to remove hair from the body. Mens’ facial hair is of course removed by shaving. Hair on other parts of the body can be shaved, and can also be removed by applying a cream designed for the purpose. The effects of unwanted facial hair in women can be lessened by applying a prescription cream called eflornithine, which slows its growth. (It interferes with an enzyme in the skin that regulates hair growth). Hair on other parts of the body can be forcefully ‘yanked out’ by plucking, or by using hot wax. Ouch!
More Hair Facts:
- People with blonde hair usually have many more strands of hair than people with red or dark hair.
- A single head hair is about 0.02mm thick, so that 20 – 50 hairs side by side make one millimetre.
- Hair has a high tensile strength. It rips only after applying a force equivalent to 60kg.
- On an average head there are about 150 hairs per square centimetre.
- Crash diets can make your hair fall out faster.