4. the integumentary system--cliffnotes a&p

8/8/2019 4. the Integumentary System--Cliffnotes a&P

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The Skin and Its Functions

The skin is far more than just the outer covering of human beings; it is an organ just like

the heart, lung, or liver. Besides providing a layer of protection from pathogens, physicalabrasions, and radiation from the sun, the skin serves many functions as well as playing a

vital role in homeostasis. This includes maintaining a constant body temperature via theacts of sweating or shivering and by making you aware of external stimuli throughinformation perceived within the touch receptors located within the integumentary

system. It only takes one visit to a burn unit to see the value of skin and the many

complications that arise when this organ is compromised.

The skin, or integument, is considered an organ because it consists of two tissues:epithelial and connective. In addition, accessory organs, such as glands, hair, and nails,

are present, and together with the skin make up the integumentary system. A section of

skin with various accessory organs is shown in Figure 1.
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Figure 1A section of skin with various accessory organs.

The skin consists of two layers, the epidermis and the underlying dermis. Althoughtechnically not part of the skin, the hypodermis (subcutaneous layer or superficial fascia)

lies beneath the dermis.

The skin performs a variety of functions:

Protection is provided against biological invasion, physical damage, andultraviolet radiation.

Sensation for touch, pain, and heat is provided by nerve endings.

Thermoregulation is supported through the sweating and regulation of blood flowthrough the skin.

Metabolism of vitamin D occurs in the skin.

Storage of blood that can be shunted to other parts of the body when needed takes

place in the skin. Excretion of salts and small amounts of wastes (ammonia and urea) occurs with

the production of sweat.

The Epidermis

The epidermis consists of stratified squamous epithelium. Four cell types are

present:

Keratinocytes produce keratin, a protein that hardens and waterproofs the skin.

Mature keratinocytes at the skin surface are dead and filled almost entirely with

keratin. Melanocytes produce melanin, a pigment that protects cells from ultraviolet

radiation. Melanin from the melanocytes is transferred to the keratinocytes.

Langerhans cells are phagocytic macrophages that interact with white blood cellsduring an immune response.

Merkel cells occur deep in the epidermis at the epidermal-dermal boundary. They

form Merkel discs, which, in association with nerve endings, serve a sensoryfunction.

Five layers make up the epidermis:

The stratum corneum contains many layers of dead, anucleate keratinocytescompletely filled with keratin. The outermost layers are constantly shed.

The stratum lucidum contains two to three layers of anucleate cells. This layer is

usually apparent only in thick skin (palms of hands and soles of feet).

The stratum granulosum contains two to four layers of cells held together by

desmosomes. These cells contain keratohyaline granules, which contribute to theformation of keratin in the upper layers of the epidermis.


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The stratum spinosum contains eight to ten layers of cells connected by

desmosomes. These cells are moderately active in mitosis.

The stratum basale contains a single layer of columnar cells actively dividing bymitosis to produce cells that migrate into the upper epidermal layers and

ultimately to the surface of the skin.

The Dermis

The second layer of the skin, the dermis, consists of various connective tissues. Asconnective tissue, it contains fibroblasts and macrophages within a gelatinous matrix

containing collagen, elastic, and reticular fibers. The structure provides strength,

extensibility (the ability to be stretched), and elasticity (the ability to return to its originalform).

The dermis consists of two layers:

The papillary layer is a thin, outer layer with fingerlike projections called dermalpapillae that protrude into the epidermis. In the hands and feet, the dermalpapillae generate epidermal ridges (sweat from the epidermal ridges leaves

fingerprints).

The reticular layer is a thick layer, below the papillary layer, that makes up most

of the dermis.

The Hypodermis

The hypodermis (subcutaneous layer, or superficial fascia) lies between the dermis and

underlying tissues and organs. It consists of mostly adipose tissue and is the storage siteof most body fat. It serves to fasten the skin to the underlying surface, provides thermalinsulation, and absorbs shocks from impacts to the skin.

Accessory Organs of the Skin

The following accessory organs (skin derivatives) are embedded in the skin:

Hairs are elongated filaments of keratinized epithelial cells that arise and emergefrom the skin of mammals. Hair is composed of the following structures:

o The hair shaft is the portion of the hair that is visible on the surface of the

skin.o The hair root is the portion of the hair that penetrates the skin (epidermis

and dermis).

o The hair follicle is the sheath that surrounds the hair in the skin.

o The bulb is the base of the hair follicle.

o The matrix is the bottom of the hair follicle (located within the bulb).

Here, cells are actively dividing, producing new hair cells. As these cellsdifferentiate, they produce keratin and absorb melanin from nearby


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melanocytes. As younger cells are produced below them, the more mature

cells are pushed upward, where they eventually die. The keratin they leave

behind contributes to the growth of the hair. The color of the hair isdetermined by the pigments absorbed from the melanocytes.

o The arrector pili is a smooth muscle that is attached to the hair follicle.

When the muscle contracts, the hair becomes erect; in humans, goosebumps are produced.

Nails are keratinized epithelial cells. The semilunar lighter region of the nail, the

lunula, is the area of new nail growth. Below the lunula, the nail matrix is activelyproducing nail cells, which contribute to the growth of the nail.

Sudoriferous (sweat) glands secrete sweat. Sweat consists of water with various

salts and other substances. There are four kinds of sudoriferous glands:

o Eccrine glands occur under most skin surfaces and secrete a watery

solution through pores (openings at the skin surface), which serve to cool

the skin as it evaporates.

o Apocrine glands occur under skin surfaces of the armpits and pubic

regions and, beginning with puberty, secrete a solution in response tostress or sexual excitement. The solution, more viscous than that secreted

by eccrine glands, is secreted into hair follicles.o Ceruminous glands secrete cerumen (earwax) into the external ear canal.

Wax helps to impede the entrance of foreign bodies.

o Mammary glands produce milk that is secreted through the nipples of the

breasts.

Sebaceous (oil) glands secrete sebum, an oily substance, into hair follicles or

sometimes through skin surface pores. Sebum inhibits bacterial growth and helpsprevent drying of hair and skin. An accumulation of sebum in the duct of a

sebaceous gland produces whiteheads, blackheads (if the sebum oxidizes), and

acne (if the sebum becomes infected by bacteria).

4. the integumentary system--cliffnotes a&p

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