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Skin forms the largest organ of the body, accounting for about 15 to 20 per cent of a person’s weight.

I. SKIN STRUCTURE
It performs many vital roles as both a barrier and a regulating influence between the outside world and the controlled environment within our bodies.

Internal body temperature (core temperature) is controlled through several processes, including the combined actions of sweat production and the rate of blood flowing through the network of blood vessels within the skin.

In very cold conditions skin blood flow drops very low, retaining heat in the centre of the body and preserving the flow of warm blood to the vital organs within the chest and abdomen and to the brain.

Skin and the fat layer beneath it also act as good insulators. When in a hot environment or when generating heat from muscle activity skin vessels open up, potentially increasing skin blood flow to as much as a third of the total output of blood from the heart. Thus the skin turns into a heat radiator. The evaporation of sweat from the skin greatly increases the rate at which heat can be lost from the body.

The physical toughness of the skin prevents the ingress of harmful chemicals and invading organisms such as bacteria and viruses. It also provides resistance to shocks for the more sensitive tissues underneath. At the same time however skin needs to be supple and be able to stretch to accommodate movement.

Skin exposed to sunlight is the main site of manufacture of vitamin D, which is essential for the growth and maintenance of our bones. The extensive network of nerves within the skin feeds information constantly to the brain concerning our surroundings. On the one hand we are thus warned of harmful extremes of temperature or of other dangers while on the other touch can be one of the most powerfully soothing and pleasurable of sensations.

Much importance is attached to the appearance of skin, especially in our modern society. Medical conditions affecting the skin can have marked effects not only on our state of well being but also on the ways we interact with other people, on our suitability for certain occupations and on the sorts of pastimes we can enjoy. Some of the consequences of skin disease such as rashes and itching may be obvious and others, such as the psychological impact, can be subtler although just as important.

The skin is made up three layers:

cau-truc-da-vn1. Epidermis:

  • This layer is seen on the surface of the skin. It is made up of cells called keratinocytes, which are stacked on top of each other, forming different sub-layers. The keratinocytes develop at the bottom and rise to the top, where they are shed from the surface as dead cells. So this layer is constantly renewing itself, the live cells changing into dead, hard, flattened cells. Melanocytes and Langerhans cells are other important cells found in the epidermis which have special functions
    • Melanocytes : These cells produce a dark pigment called melanin which contributes to skin color and provides UV protection. They are located at the bottom of the epidermis.
    • Dendritic (Langerhans) cells : These cells are involved in the epidermal immune system. They engulf foreign material that invades the epidermis and migrate out of the skin to stimulate an immune response.
    • Basal cells : Small cells found at the bottom of the epidermis. Earlier it was believed that basal cell carcinoma is derived from these cells. As of this writing basal cell carcinoma is thought to arise from non-differentiated cells from the basal cell layer.
    2. Dermis
    The dermis consists mostly of connective tissue and is much thicker than the epidermis. It is responsible for the skin’s pliability and mechanical resistance and is also involved in the regulation of the body temperature. The dermis supplies the avascular epidermis with nutrients by means of its vascular network. It contains sense organs for touch, pressure, pain and temperature (Meissner´s corpuscles, Pacinian corpuscles, free nerve endings), as well as blood vessels, nerve fibres, sebaceous and sweat glands and hair follicles.
    • Blood Vessels : These are tiny pipes through which blood circulates. The blood vessels supply the skin with fresh blood, which contains nutrients and oxygen, and carry away waste products.
    • Meissner’s corpuscle : These touch receptors are expecially effective in detecting light touch and soft, fleeting movements.
    • Pacinian corpuscles : Pacinian corpuscles function as receptors for deep pressure and vibration.

      Free Nerve Endings : Free nerve endings are sensitive to pain, temperature changes and itchiness.

      Nerve Fibers : Nerve fibres forward information.

    • Sebaceous Glands : Sebaceous or oil glands are small, sacculated organs that secrete sebum. This oily substance is a natural moisturiser which conditions the hair and skin. Sebaceos glands are found all over the body, but they are more numerous in the scalp area and around the forehead, chin, cheeks and nose.
    • Sweat Glands : These are sweat-producing structures consisting of a single tube, a coiled body and a superficial duct. They are involved in thermoregulation, as they cool the skin by sweating.
    • Hair Follicles : Hair follicles are downward growths into the dermis of epidermal tissue and produce hair. They are found all over the body except on the palms of the hands and soles of the feet as well as on the lips. When the body gets cold, the hair stands upright with the help of the arrector pili muscle, closing up the skin’s pores and keeping the warmth in.
    • Arrector pili muscle : This small muscle is attached to the base of the follicle. When it is stimulated by cold or fright, it pulls the hair follicle up, causing it to stand upright.
    3. Subcutaneous layer
    The subcutaneous layer below the dermis consists of loose connective tissue and much fat. It acts as a protective cushion and helps to insulate the body by monitoring heat gain and heat loss. Not all authors consider this layer a part of the skin, but it definitely has a strong impact on the way the skin looks.
 

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