What is the function of hair?

Why do we have hair?

The reasons we have hair are not completely understood. We do know that hair, along with skin pigmentation, is the primary natural protector we have against the sun’s ultra-violet rays.  The hair on your head helps prevent trauma to the skull.  Heck, it can even save your life!

Hair also helps to insulate and conceal.  On other parts of your body it acts as a “dry lubricant” in areas that rub, such as in the arm pits or groin areas, and hair disperses pheromones (body secretions that are involved in sexual attraction.)

Another function of hair is to sense the immediate surroundings, such as how a cat uses its whiskers. Although human hair doesn’t have as many sensory nerve fibers in their roots like a cat, we do have a nerve fiber running to the bulb of each hair follicle.  Movement of a hair causes a sensation that makes you aware of something on your skin’s surface, such as when a mosquito walks on your arm.

In the animal world hair is a major defense mechanism. When an animal is startled its fur bristles, which makes the animal appear larger and more menacing. You’ve probably seen this on an upset cat, when its tail poofs up, or when a dog raises the hackles on their neck. An extreme example is the porcupine. Their quills are actually modified hairs which can be used as a weapon. What causes the hairs to stand on end is a tiny muscle, called the erector pili, which connects the lower portion of each hair shaft with the underside of the skin. When an animal or person is frightened or angry these small muscles contract and cause the hair to stand on end.  Just like when you get goosebumps.

What exactly is hair anyways?

Anatomically, hair is a distinct part of the skin referred to as an appendage. Other skin appendages include sweat glands, fingernails and toenails. Skin is composed of three main layers. The outer layer of skin is the epidermis which is less than a millimeter in thickness and is composed of dead cells that are in a constant state of sloughing and replacement. As dead cells are lost, new ones from the growing layer below replace them.

Beneath the epidermis is the dermis, a tough layer of connective tissue (collagen) that is about 2 to 3 mm thick on the scalp. This layer gives the skin its strength, and contains both sebaceous glands and sweat glands. Beneath the dermis is a layer of subcutaneous fat and connective tissue. The larger sensory nerve branches and the blood vessels that nourish the skin run deep in this layer. In the scalp, the lower portions of the hair follicles (the bulbs) are found in the upper part of this fatty layer.

An interesting characteristic of hair is that, in contrast to the commonly held notion that it grows as individual strands, it actually emerges from the scalp in groups of one to four (and sometimes even five or six). The reason for this is that hair follicles are not solitary structures, but are arranged in the skin in naturally occurring groups called follicular units.

Each hair follicle measures about 3-4 mm in length and produces a hair shaft about 0.1 mm in width. Each hair shaft also contains a small gland called the sebaceous gland, located next to the hair shaft. Sebaceous glands make a yellow, fatty substance called sebum that lubricates the hair. Each time the erector pili muscle contracts, the gland is squeezed, and a small amount of lubricant is applied to the surface of the hair. The hair follicle has five main parts. Starting from the bottom of the follicle, they are; the dermal papillae, matrix, outer root sheath (ORS), inner root sheath (IRS), and the hair shaft.

The dermal papillae contains specialized cells called fibroblasts that regulate the hair cycle and hair growth. They also contain androgen receptors that are sensitive to DHT. For many years, scientists thought that hair growth originated from the dermal papillae. Recent evidence has shown that the growth center extends from the dermal papillae all the way up to the region of the follicle where the sebaceous glands are attached. It is now believed that the primary function of the dermal papillae is to regulate follicular growth and differentiation. If the dermal papillae is removed the hair follicle is often able to regenerate a new one, although the growth of the new hair will be delayed.

The matrix sits over the dermal papillae and contains actively dividing, immunologically privileged cells. Together the dermal papillae and the matrix are referred to as the hair bulb. The size of the bulb and the number of matrix cells will determine the width of the fully-grown hair. The cells of the matrix differentiate into the three main components of the hair follicle: outer root sheath, inner root sheath, and hair shaft.

The outer root sheath or trichelemma (Greek for coating sac), surrounds the hair follicle in the dermis and then blends into the epidermis on the surface of the skin, forming the structure commonly referred to as the pore from which the hair emerges.

The inner root sheath essentially forms a mold for the developing hair shaft. It is composed of three parts (Henley layer, Huxley layer, and cuticle), with the cuticle being the innermost portion that touches the hair shaft. The cuticle of the inner root sheath is formed by a layer of overlapping cells that interlock with the cuticle of the hair shaft. This overlapping mechanism holds the hair shaft securely in place, but also allows it to grow in length. The cells of the inner root sheath keratinize giving it rigidity and strength. Racial variations are felt to be due to the asymmetric formation of the inner root sheath. If you look at the cross section of the inner root sheath, the shape is oval in Europeans, flat in Africans, and round in Asians.

The hair shaft is the only part of the hair follicle to exit the epidermis (the surface of the skin). The hair shaft itself is also composed of three layers. The cuticle, which is the outer layer that interlocks with the internal root sheath, forms the surface of the hair and is what we see as the hair shaft emerges from the follicle. The middle layer, the cortex, comprises the bulk of the hair shaft and is what gives hair its strength. It is composed of an organic protein called keratin, the same material that comprises rhinoceros horns and deer antlers. The center, or core, of the hair shaft, is the medulla, and is only present in terminal hair follicles.
The normal human scalp contains between 100,000 to 150,000 follicles that produce thick terminal hair. These hairs do not emerge individually from the scalp, but are arranged in follicular units, small groups of 1 to 4 hairs each. There are approximately 50,000 to 65,000 follicular units on the human scalp. For comparison, the human body has approximately 5 million follicles that produce the fine, vellus hair.

At any given time, about 90% of terminal hairs on one’s head are actively growing. This phase, called anagen, can last from 2 to 7 years, though the average is about three years. In catagen, which is the shortest phase lasting about 2-3 weeks, growth stops, the middle of the follicle constricts and the lower portion expands to form the “club.” The other remaining 10% of scalp hairs are in a resting state called telogen that, in a normal scalp, lasts about 3 to 4 months.

When a hair enters its resting phase, growth stops, and the bulb detaches from the papilla, and the shaft is either pulled out (as when combing one’s hair) or pushed out when the new shaft starts to grow. When a hair is pulled out, or falls out on its own, a small, white swelling is found at the bottom of the hair shaft. Most people assume that this is the growth center of the hair, but it is just the clubbed, detached lower end of the hair shaft. The dermal papillae and the growth center of the hair remain in the scalp. Scalp hair grows at a rate of about 0.44 mm/day (or 1/2 inch per month). Each hair follicle goes through the hair cycle 10-20 times in a lifetime.

Additional Information

10 Responses to “What is the function of hair?”

  • Felice from anti aging:

    Oh.. Its think it is only a miracle when her hair make the bullet stops from penetrating to the brain.

  • Don from Laser Eye Surgery:

    A friend of mine had very severe hairloss at the age of 26, I believe it was related to a hormonal imbalance as she also suffered from fertility related concerns. Such a shame at such an early age.

  • Hair loss is quite traumatic, especially for women. I had a friend who became seriously ill in her early 20’s and her hair became extremely thin. She wore a wig for years, then started having weaves put into her hair. Very time consuming and expensive process, but it helped make her feel better about herself. People want their hair to look good no matter what they have to do. It can seriously damage your self esteem when something damages your hair.

  • Jane is right. Our culture doesn’t accept baldness very well for anyone, but especially for women. It’s unfortunate that we have that societal bias– balding is so common that social reprecussion reach many.

  • Jamal from Human Hair Weave:

    I have a friend, who suffered premature hair loss in his twenties. In their thirties he was almost completely bald. Now he wears a wig. Looks just like real hair. But I hope someday they will invent something to prevent hair loss.

  • Jane from Ladies Wigs:

    Society does not accept baldness in women as easily as it does men. I hope the research works out soon and leads to a permanent cure.

  • John Michaels from buyhumangrowthhormonepills.com:

    I used to worry about my hair loss until I read a few articles that some bald men are considered very hanson. I now believe if you have thinning hair “less is more”. I keep my hair very short 1/8 of an inch and it looks good.

  • I cannot even imagine going bald as a women. I didn’t realize it was that common in women. Interesting to think what I can do now to help prevent it later. I hope it wont ever be a problem for me.

    Good to know there is actually research and products continually improving. Maybe by the time im 40 they will have product that stops balding all together.

  • Hair loss was a feature of all my male relatives, father, grandparents and uncles on both my father’s side and my mother’s side of the family. I have an elder brother, who suffered premature hair loss in his twenties. I grew up expecting it with simple resignation.

    I wish it could have been otherwise.

    I jokingly said I would like to keep it until it went grey – and I did, but only because I was prematurely grey. I look at photos of my children’s baby years and see a man with thinning hair, and my wife says I have changed much over the years – but realy she means the receding and disappearing hairline. Look at your friends old photos – if they have lost their hair, the most striking thing you will notice and comment upon will be the hair they once had.

    I would very much have liked to have kept my hair, for simple practical reasons – it provides a layer of warmth in winter, and a sun shield in summer. And a well groomed head of hair always looks smarter than a glistening dome.

    It would have been great to have been aware of the opportunities to treat premature baldness at a time when it could have been slowed and perhaps prevented. It is a much better to take action to prevent baldness, than face the prosepct of reversing what has happened. Reading just how complex hair is, makes it all the more compelling to take care of your hairline in good time
    .-= Brian@ cordlesshairtrimmer.org´s last blog ..Contact Us =-.

  • Romy from skin care products:

    I think a lot of people has problems with losing hair, but don’t really pay attention to it until is too late. Even I lose hair, but I guess that being young helps, as it grows back a lot faster.

    And even though i tried several products to stop it from falling, none worked, so far. I guess I’ll keep trying.

    Interesting fact you have here, a lot of good information. I’ll have to read some more, maybe I’ll find something that could help me.
    .-= Romy @ skin care products´s last blog ..Do follow, comment luv keywordluv =-.