Skin & Hair
Skin does far more than keep our insides in and make us look good. It gives us the sense of touch, helps to control body temperature, protects us from knocks and damage, replaces itself to cope with wear and rubbing and keeps out dirt, dangerous germs, and harmful rays.
Skin is the largest single part, or organ, of the human body. On a typical adult, it has an area of about two square metres and weighs almost four kilograms. It varies in thickness from about 0.5 millimetres on delicate parts such as the eyelids, to 5 millimetres or more on the soles of the feet.
OUTER SKIN LAYER
There are two main layers of skin. The outer one is the epidermis. It has several layers of microscopic cells. The cells at the base are always multiplying to make more cells. As these pass outwards they become flatter and fill with a tough body substance known as keratin, which is also present in fingernails and toenails. By the time the cells reach the surface they are flattened and dead and look like tiles on a roof.
The skin’s surface is always being worn away by activities such as moving about, wearing clothes, washing, and rubbing. On average, 50,000 cells flake away every second. There are always more cells coming up from below to replace them. The whole outer layer of skin is replaced about once every month.
On the palms, fingers, soles, and toes, the epidermis has swirly patterns of ridges. These help to give better grip. The patterns are different for everyone in the world. On the fingers, we call them fingerprints.
Among the cells at the base of the epidermis are melanocyte cells. These have a spidery shape and are specialized to make tiny particles of a coloured substance known as melanin. The more melanin they make, the darker the skin colour. If the skin is exposed to strong sunlight, the melanocytes gradually make more melanin. This darkens the skin to protect it against the Sun’s harmful ultraviolet rays. The darkening is known as a suntan. If the exposure to strong Sun is too fast, the skin becomes red, swollen and painful, and this is known as sunburn. Freckles are brown spots that contain an excess of melanin. They are usually seen on the faces of fair or red-haired people and they come more obvious when someone has been out in the sun.
The effect of melanin also explains how people can inherit skin colour from their parents. People whose ancestors lived in a hot, sunny climate are likely to have darker skin than those whose ancestors lived in a cold, dark climate.
INNER SKIN LAYER
The inner layer of skin is the dermis, and this lies under the epidermis. It contains millions of micro-fibers of two body substances: tough collagen for strength, and stretchy elastin for flexibility. The dermis also contains:
Tiny blood vessels are known as capillaries. A thumbnail-sized patch of skin has more than three metres of capillaries.
Micro-sensors for touch. Some of these are near the top of the dermis and detect very light contact. Others are buried deeper and respond to heavy pressure. Still, others detect heat, cold and pain. Sensitive skin, such as on the fingertips, has hundreds of these micro-sensors in a patch of skin the size of this o.
Sweat glands. Each sweat gland is a tiny coiled tube in the dermis, with a straighter tube or duct leading to an opening on the surface known as the sweat pore. An average person has three million sweat glands. If all their tubes were straightened out and joined end to end, they would stretch for 50 kilometres.
Hair follicles. These are tiny pockets or pits in the dermis, from which hairs grow. There are hairs all over the body—10 million or more—but most are too short and thin to notice. They are described in more detail below.
Sebaceous glands. There is a sebaceous gland for each hair follicle. They make sebum, which is a natural waxy, oily substance that keeps skin supple and semi-waterproof.
Skin is important in controlling body temperature. If the body is too cold, its blood vessels become narrower. Less blood flows through the blood vessels, and since blood carries body heat, this means less heat is lost through the skin’s surface. It also makes the skin look pale. In addition, each hair is pulled upright by a tiny muscle known as the arrector pili, which is attached to its base. The upright hairs help to trap a layer of air next to the skin, which also reduces heat loss. (The muscles push the skin into small mounds called “goosebumps”.) The sweat glands release less sweat, which saves warmth too.
If the body is too hot, the reverse happens to when it is too cold. The skin’s blood vessels widen, allowing more blood to flow and more heat to be lost. This also makes the skin look reddened or flushed. Hairs lie flatter, trapping less air. Sweat glands pour their sweat onto the surface. As this dries it draws more heat from the body.
WHAT ABOUT HAIRS?
Each hair grows from a pit-like follicle in the dermis. It grows at the base or root only. The microscopic cells of the hair become flattened, filled with keratin, dead and glued together. This forms the main length or shaft of the hair.
Most people have about 100,000 hairs on their head, and these are called scalp hairs. These grow by up to two centimetres every month. Each hair lasts for up to five years. Then it falls out, but it is usually replaced quickly by a new hair growing from the same follicle. This natural loss happens at different times for different hairs. It is why, each time we use a brush or comb, a few hairs always come out.
For many people, particularly older adults, the rate of hair loss is greater than the rate at which it can be replaced. Hair starts to look thinner. Eventually, in most men, hair finally stops growing and so they become bald.
HAIR AND SKIN HEALTH
Every day, dust, dirt, and germs stick to skin and hair. They need to be cleaned away regularly with warm water and soap. Otherwise, the skin may suffer from infections and sores, and also become smelly.
The skin can develop several very irritating diseases and conditions: dermatitis and eczema are skin inflammations; sebaceous cysts are fluid-filled sacs of sebum; ulcers are sores of destroyed skin; acne is an eruption around a hair follicle; psoriasis can cover the skin in blotches; seborrhea is a disease of the sebaceous glands; warts are caused by the human papillomavirus; and boils are infections that create pus-filled swellings in hair follicles or breaks in the skin. Birthmarks contain an excess of blood capillaries.
ANIMAL HAIR AND SKIN
Only mammals like us have hair. On many of them, the hairs grow thickly over most of the body and are called fur. On some mammals, certain hairs are thick and stiff, like a hedgehog’s spines or porcupine’s quills. Other mammals, such as hippopotamuses and whales, have just a few bristly hairs here and there. The toughest skin belongs to the rhinoceros—in some places, it is more than 10 centimetres thick.