Skin is a thin layer of tissue forming the natural outer covering of the body of a person or an animal. It is the largest organ of the body. The skin protects us from microbes and the elements helps regulate body temperature, and permits the sensations of touch, heat, and cold. The skin’s color is created by special cells called melanocytes, which produce the pigment melanin. Melanocytes are located in the epidermis.
Structure of the Skin
The mammalian skin consists of two main layers: the epidermis, and the dermis.
Epidermis: is the outermost layer of the skin. It provides a waterproof barrier and creates our skin tone. The epidermis consists of three layers:
· The innermost Malpighian layer
· The middle Granular layer, and
· The outermost (surface) Cornified layer.
1. Malpighian Layer: Also known as germinative layer, consists of actively dividing cuboidal cells. They contain melanin, a pigment that gives the skin its colour and absorbs ultra violet radiation. The cells of this layer get their nutrients and supply of oxygen by diffusion from the blood of the capillaries found in the dermis.
2. Granular Layer: This consists of living cells produced by the malpighian (germinative) layer beneath. These cells are continuously converted to cornified cells. Keratin is deposited inside them, and they lose their nuclei and become flattened in shape.
3. Cornified Layer: This consists of scale-like dead cells impregnated with keratin. The keratin makes this layer tough, flexible and waterproof. They are constantly wearing away and are replaced from the granular layer beneath.
Dermis: This is beneath the epidermis. It is a layer of connective tissue containing blood capillaries, hair follicles, sweat glands, sebaceous gland, sensory nerve ending and fat cells.
1. Blood capillaries: This supply food and oxygen to the dermal and epidermal cells and remove wastes. The capillary loops close to the body surface help to regulate the body temperature.
2. Hair Follicles: This is a deep pit formed by the infolding of the Malpighian layer. Each hair is a cylinder composed of dead cells; grow as new cells at the ‘root’. A hair erector muscle is attached to each follicle. Its contraction pulls the hair to a more upright position, i.e. it makes the hair ‘stand up’.
3. Sebaceous glands: Secrete sebum which repels water (waterproof) and also prevents microbes from multiplying.
4. Sweat gland: Absorb fluid from the surrounding tissues and capillaries. This fluid is then passed out as sweat through the sweat duct. Sweat is 99% water, 0.3% salt, and minute amounts of urea and lactic acid.
5. Sensory nerve ending: the skin is also sense organ. Various nerve endings, capable of responding to touch, heat, cold and pressure.
6. Subcutaneous fat: It is found beneath the dermis. It acts as a food reserve and an insulating layer, to prevent heat loss.
Functions of the skin
1. It protects the body against dehydration, invading microbes, mechanical damage and damage due to ultra violet rays and poisonous chemicals,
2. It contains receptors sensitive to heat, cold, touch and pressure.
3. It plays a major role in temperature control (vasodilation and vasoconstriction).
4. It has a minor role as an excretory organ. Urea and lactic acid are lost.
5. The skin produces vitamin D in the fatty cells by using infra-red rays of the sun.
Diseases of the skin
The diseases of the skin may be caused by viruses, bacteria, protozoa or fungi. These include:
Ø Chicken pox
Ø Skin rashes
Ø Boil etc.
Care of the skin
Ø Take bath twice daily.
Ø Eat balanced diet.
Ø Take abundant fresh air and regular exercise.
Ø The use of anti-perspirants and deodorants may control the excessive sweating and unpleasant odour.
Control of Body Temperature
Under normal conditions, the heat the body gain is balanced by the heat it loses. The balance, however, can be upset by hot weather, vigorous exercise, high fever or exposure to solar radiation. The balance is restored by the actions of the hypothalamus and the skin.
· A rise in body temperature stimulates the following processes to get rid of excess body heat:
1. Vasodilation: The arterioles in the skin (swell) so increasing the flow of blood through the skin. This leads to increased loss of heat through the dermis by convection and radiation.
2. Sweating: The sweat glands become active and produce large amounts of sweat that flow out onto the surface of the skin. As this sweat evaporates, heat from the body is used up, thus cooling the body.
· A fall in body temperature stimulates the following processes tp produce and conserve heat:
1. Vasoconstriction: The arterioles are narrowed, thereby reducing the flow of blood to the skin and so minimizing heat loss.
2. Sweating: The sweat glands become inactive and produce very little sweat that flows out to the skin surface, thereby conserving body heat.
TESTS AND EXERCISES
1. The functions of the skin include all of these except __________________. (a) taking in nutrients (b) preventing the entry of microorganisms (c) retarding water loss (d) regulating body temperature Answer: Taking in nutrients
2. Heat loss occurs from the body by each of these methods except __________________. (a) radiation (b) convection (c) cellular respiration (d) conduction Answer: Cellular respiration
3. What is one important function for melanin? (a) strengthen the skin or eyes (b) cause the skin to be cooler (c) increase the amount of blood vessels (d) absorb harmful radiation Answer: Absorb harmful vessels
4. The part of the hair below the surface of the skin is called the _____. (a) follicle (a) root (c) shaft (d) base Answer: Root
5. The protective dead layer of cells of the outer epidermis is called the _______________. (a) malpighian layer (b) granular layer (c) cornified layer (d) sweat gland Answer: Cornified layer
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