Skin

What is the skin?
The skin is one of the largest organs of the body. It has an average surface area of about 2 square meters. It forms a protective barrier between the outside environment and the internal environment of the body that contains the internal organs, muscles, blood vessels and nerves. The hair and nail growing from the skin provide extra protection. The color of the skin varies enormously between different races of mankind. The texture changes with changes in age and illness. Hence the skin is an indicator of the general health of the body.

Where is the skin situated?
The skin covers the whole of the external body from head to toe. The texture of skin varies at different areas on the body. At different levels the skin changes gets gradually transformed into other types of protective layers such as the mucus membranes. These areas of transition of the skin are the mouth, lips, nostrils, anus, labia in the females and the urinary opening in the males.

How can we describe the skin?
The skin is a living organ. The uppermost layer of the skin is called as the epidermis. This layer is made up of dead cells. The body sheds about 30,000 of these dead cells every day. Live cells are continuously produced in the lower layer of the epidermis to replace these dead cells. The layer beneath the epidermis is called as the dermis. This layer of the skin contains the nerve endings and glands of the skin such as sweat glands and sebaceous glands. The layer beneath the dermis is the fat layer, which insulates the body and acts as a shock absorber. This fat is also utilized when energy is required by the body, thus acting as an energy store.

The main component of the surface of the skin is a tough, fibrous protein called as keratin. The skin contains numerous sweat and sebaceous glands. Sweat glands in the skin secrete sweat, which helps to maintain the temperature of the body. The skin also contains sebaceous glands that produce an oily fluid called as sebum. This oily fluid makes the skin supple and waterproof.

Lets look at he layers of the skin in some more detail:

• The epidermis
  This is the outer thin layer of the skin and consists of sheets of tough flat cells. The epidermis contains ducts of the sweat glands, sebaceous glands and their openings or pores on the surface. The upper layer of the epidermis contains dead, scaly cells. Structures called as papillae lie between the epidermis and the dermis. A layer of cells called as the basal cell layer also lies between the epidermis and dermis. New cells are produced in this layer. In most areas of the body the epidermis has four layers of cells.
  These are:
  1. The dead cell layer
  2. The granular cell layer
  3. The prickle cell layer that firmly anchors the skin and thus strengthens the skin
  4. Basal cell layer
• The dermis
  The dermis is a thick layer beneath the epidermis. It contains numerous structures, which are as follows:
  • Blood vessels consisting of arterioles and venules
  • The sweat glands.
  • The sebaceous glands.
  • The hair bulb and hair follicle from which the hair grows. Nerve receptors sensitive to touch are also present in this layer.
  • The erector pili muscle, which tenses to pull the hair upright.
  • Free nerve endings that respond to touch, cold, heat and pain.

The skin gets its color from cells that produce a pigment called as melanin. These cells are called as melanocytes. Melanin is produced in response to sunlight; hence more the exposure to sunlight, darker is the skin. This pigment filters the ultra-violet rays of the sun. Thus the skin becomes less susceptible to sunburn.

Functions of the Skin

Protection
Keratin, which is also found in hair and nails, provides warmth, shape and strength to the hair and nails. The skin forms a highly effective barrier against a variety of microorganisms and harmful substances. Wounds in the skin allow growth of bacteria and weaken the protective function of the skin.

Repair
The ability of the skin to repair itself, even when damaged is most useful in defending the body against foreign organisms. Repair of skin is carried out systematically over a period of a few days depending on the severity of the wound. Initial bleeding in the wound produces a clot that stops further bleeding. Soon after there is influx of special cells like fibroblasts and repair cells into the wound. These cells multiply and migrate to the damaged area. Fibroblasts produce strands of fibrin and form a framework for the new cells to build onto. New skin begins growing under this framework of fibrin. The fibrin gradually becomes a scab and falls of when new skin production is complete.

Sensory Functions
There are receptors in the skin that are responsible for our sense of touch. These receptors are special organs that can sense changes in the pressure, temperature, vibrations and pain. These receptors send information continuously to the brain where it is processed and learnt. These receptors then warn us of dangers to the skin such as a hot stove. Some areas of the skin have a low density of these receptors like the back, chest and the thighs. Some areas like the fingers and the back of the hand have a high density of the receptors. These areas of the skin are highly sensitive to touch.

Thermo-Regulation
The skin plays a major role in regulating body temperature. If the body is too hot then there is dilation of the blood vessels that allows more blood to reach the surface of the skin where it cools. Sweat is also carried to the surface carrying the body heat and is evaporated from the skin. If the body is too cold, then the blood vessels on the surface contract; the erector pili muscles also contract and pull hair erect to create goose bumps. These goose bumps help to trap a layer of air that act as insulation for the skin.

Production of vitamin D
On exposure to sunlight, skin stimulates the production of vitamin D in the body, which is essential for the development of healthy bones.

Adaptation to work style
The skin of our body gets adapted to the nature of our work. Hence a gardener has coarse skin on his hands. This thickened skin provides extra protection against coarse soil and instruments.