Skin

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SKIN
Largest and heaviest organ of the body; accounts for about 15% of the total body weight •Also called as cutaneous membrane
Skin contains two layers, the epidermis and the dermis.
The hypodermis is the layer underneath the skin.

FUNCTIONS
Skin is a protective covering that prevents harmful substances from entering the body. •It helps regulate body temperature and water loss.
It houses sensory receptors and contains immune system cells. •It synthesizes chemicals and excretes some wastes.

HAIR FOLLICLES
Hair develops from epidermal cells at the base of the hair follicle. •Epidermal cells keep dividing continuously, grow, and die. •The dead cells make up the shaft of the hair.
The arrector pili muscle is a smooth muscle attached to the follicle. •It causes hair to stand upright on the skin.

Hair is filament of keratinized cells
shaft is visible above skin; root is below within follicle FUNCTIONS OF HAIR
1.Sensation
2.Communication
Facial, pubic and auxiliary hair
Eyebrows
3.Protection
Barrier
4.Thermoregulation

THE EPIDERMIS
It is composed of stratified squamous epithelium (ET) and a simple basal layer of cuboidal cells. •It lacks blood vessels, but the deepest layer, the stratum basale, is close to the dermis and receives nutrients. •The thickness of the epidermis varies with region of the body.

CELLS OF EPIDERMIS
Keratinocytes (90%)- waterproofs & protects skin, nails, hair, stratum corneum •Melanocytes (8%)- produce melanin
Merkel Cells- slow mechanoreceptors
Langerhans’ Cells- immunological defense

THE DERMIS
Thickness = 0.6mm to 3mm
Composed mainly of collagen, but also contains elastic reticular fibers, blood vessels, sweat glands, sebaceous glands, hair follicles, nail roots, sensory nerve endings and muscular tissue (facial expressions are due to the skeletal muscle connection to the dermal collagen fibers to producesmile, frown, eyebrow movement) •Functions: Pressure detection; metabolism (duplication of cells)

THE HYPODERMIS
Known as subcutaneous tissue or superficial fascia made up of loose, fibrous tissue, rich in blood vessels, lymphatic vessels and nerves •Has more adipose than the dermis
Functions:
energy reservoir
thermal insulation

TYPES OF RECEPTORS
Mechanoreceptors: These receptors perceive sensations such as pressure, vibrations, and texture. •Thermoreceptors: These receptors perceive sensations related to the temperature of objects the skin feels. •Pain receptors: These receptors detect pain or stimuli that can or does cause damage to the skin and other tissues of the body. •Proprioceptors: They sense the position of the different parts of the body in relation to each other and the surrounding environment.

TYPES OF MECHANORECEPTORS

Sensory receptors are connected to interneurons, which pass the electrical impulses generated by the receptor to the central nervous system (the brain and the spinal cord). The brain receives and makes sense of the information and directs our response to the stimulus. Responses can occur without input from the brain, an example is the reflex that causes you to withdraw from a painful stimulus, such as a hot object. This response involves input from pain receptors, a relay through interneurons in the spinal cord and output to the muscles. However, even simple reflexes can be modified by the brain. For example, the brain can shut down the withdrawal response in order to prevent you from dropping a hot pot.

PROBLEMS IN TEMPERATURE REGULATION
Hypothermia or lowered body temperature is a dangerous condition resulting from cold exposure. •Hyperthermia or increased body temperature can result from inadequate body cooling during exercise or in extreme heat.

SWEAT GLANDS
Sweat or sudoriferous glands are small tubular structures of the skin that produce sweat. •There are two kinds of sweat glands:...
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