(skin & its appendages)The skin covers the entire surface of the

human body. In an adult, the skin has a surface area of about 1.8 square meters

Principal functions:

Protection, water retention, thermoregulation, vitamin D synthesis, sensation, nonverbal communication and has also limited excretory and absorbing powers.

The skin (integument) is the largest organ of the body, and together with its accessory organs (hair, glands, and nails), it constitutes the integumentary system

In certain areas of the body, it has adaptive modifications that accommodate protective or metabolic functions.

In its role as a dynamic interface between the continually changing external environment and the body’s internal environment, the skin helps maintain homeostasis.

Structure of the Skin

consists of two regions:

the epidermis

the dermis.

The hypodermis;

a subcutaneous tissue, is found between the skin and any underlying structures, such as muscle.

But where no muscles are present, the hypodermis attaches directly to bone or underlying deep fascia, e.g. there are flexion creases where the skin attaches directly to the joints

The epidermis is the outer and thinner region of the skin.

It is made up of stratified squamous epithelium divided into several layers;

the deepest layer is the stratum basale,

the most superficial layer is the stratum corneum.

Epidermis

DermisThe dermis, is a deeper and thicker region

than the epidermis, is composed of dense irregular connective tissue.

The upper layer of the dermis has fingerlike projections called dermal papillae.

Dermal papillae project into and anchor the epidermis.

In the overlying epidermis,dermal papillae

cause ridges, resulting in spiral patterns

commonly known as “fingerprints’.

The function of the epidermal ridges is to

Increase friction to provide a better ‘grip’.

Because they are unique to each person,

fingerprint and footprints can be used for

identification purposes.

The dermis contains :

collagenous and elastic fibers.

The collagenous fibers are flexible but offer great resistance to overstretching;

So they prevent the skin from being torn.

elastic fibers stretch to allow movement

of underlying muscles and joints, but they

maintain normal skin tension.

The dermis also contains blood vessels that nourish the skin.

Sometimes, blood flow to a particular area is restricted in bedridden patients, and consequently they develop decubitus ulcers (bedsores).

There are also numerous sensory nerve fibers in the dermis.

The Appendages of the Skin:The appendages of the skin are:

the nails

the hairs

the sudoriferous (sweat glands) , and

sebaceous glands with their ducts.

Sweat Glands;

A sweat gland is tubular. The tubule is coiled, particularly at its origin within the dermis.

These glands become active when a person is under stress.

Two types of sweat glands are present:

Apocrine glands open into hair follicles in the anal

region, groin, and armpits. These glands begin to secrete at puberty.

The anatomical position is described as follows:

Eccrine glands open onto the surface of the skin.

They become active when a person is hot, helping

to lower body temperature as sweat evaporates.

The sweat (perspiration) produced by these glands

is mostly water, but it also contains salts and some

urea. Therefore, sweat is a form of excretion.

Ears contain modified sweat glands, called Ceruminous

glands, which produce cerumen (earwax).

Sebaceous GlandsMost sebaceous glands are associated with a hair

follicle. These glands secrete an oily substance called sebum that flows into the follicle and then out onto the skin surface. This secretion lubricates the hair and skin, and helps waterproof them.

Particularly on the face and back, the sebaceous glands may fail to discharge sebum, and the secretions collect, forming whiteheads or blackheads.

If pus-inducing bacteria are also present, a boil or pimple may result.

Acne vulgaris is the most common form of acne, is an inflammation of the sebaceous glands that most often occurs during adolescence.

Hormonal changes during puberty cause the sebaceous glands to become more active at this time.

Hair and Nails

Hair is found on all body parts except the

palms, soles, lips, nipples, and portions of the

external reproductive organs.

Most of this hair is fine and downy, but the

hair on the head includes stronger types as

well.

After puberty, when sex hormones are made in an enough quantity, there is noticeable hair in the axillary and pelvic regions of both sexes.

In the male, a beard develops, and other parts of the body may also become quite hairy.

When women produce more male sex hormone than usual, they can develop hirsutism.

In males, baldness occurs when the hair on the head fails to regrow.

Alopecia, meaning hair loss, can have many causes.

Hairs project from complex structures called hair follicles. These hair follicles are formed from epidermal cells but are located in the dermis of the skin.

Certain hair follicle cells continually divide, producing new cells that form a hair.

At first, the cells are nourished by dermal blood vessels, but as the hair grows up and out of the follicle, they are pushed farther away from this source of nutrients, become keratinized and die.

The portion of a hair within the follicle is called the root,

and the portion that extends beyond the skin is called the shaft.

Each hair has one or more sebaceous, glands,

whose ducts empty sebum into the follicle (oil).

A smooth muscle, the arrector pili, attaches to the follicle in such a way that contraction of the muscle causes the hair to stand on end. If a personhas had a scare or is cold, “goose bumps” develop due to contraction of these muscles (gooseflesh).

NailsNails grow from special epithelial cells

at the base of the nail in a region called the nail root

(a portion where a nail is implanted to the skin),

These cells become keratinized as they grow out over the nail bed.

The visible portion of thenail is called the nail body,

the distal extremitythe free edge.

The part beneath the body androot of the nail is called the nail matrix

or .

NAMESThe cuticle is a fold of skin hiding

the nail root. Ordinarily, nails grow only about 1 millimeter

per week (fingernails faster than toenails).

The pink color of a nail is due to the vascular dermal tissue beneath it.

The lunula is the whitish color, half-moon shaped base of the nail results from the thicker germinal layer in this area.

Lines of tension:

Lines of tension are

caused by the pull of

elastic and

collagenous fibers

within the dermis of

the skin.

Surgical incisions

made parallel to the

lines of tension heal

more rapidly and

create less scar

tissue than those

made across the lines

of tension.

THE END