Anatomy & Physiology
Chapter 7: Skin and Its Appendages
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Introduction
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Skin (integument) is body’s largest
organ
Skin measures approximately 1.6 to
1.8 m2 in average-sized adult
Integumentary system describes the
skin and its appendages—the hair,
nails, and skin glands
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Structure of the Skin
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Skin classified as cutaneous membrane
Two primary layers—epidermis and dermis;
joined by dermoepidermal junction (Figures 7-1
and 7-2)
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Hypodermis lies beneath dermis
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Thin and thick skin (Figure 7-3)
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“Thin skin” —covers most of body surface (1 to 3
mm thick); has hair and smooth surface
“Thick skin”—soles and palms (4 to 5 mm thick);
ridged surface with no hair
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Structure of the Skin
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Epidermis
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Cell types (Figure 7-4)
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Keratinocytes—constitute over 90% of cells present; principal
structural element of the outer skin; sometimes called
corneocytes after they are fully keratinized
Melanocytes—pigment-producing cells (5% of the total);
contribute to skin color; filter ultraviolet light
Epidermal dendritic cells—branched antigen-presenting cells
(APCs); they play a role in immune response; also called
Langerhans cells
Tactile epithelial cells (Merkel cells)—attach to sensory nerve
endings to form “light touch” receptors
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Structure of the Skin
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Epidermis (cont)
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Cell layers
Stratum basale (base layer)—single layer of
columnar cells; only these cells undergo mitosis
and then migrate through the other layers until
they are shed; stratum germinativum (growth
layer) is another name for stratum basale (or
stratum spinosum and stratum basale together)
 Stratum spinosum (spiny layer)—cells arranged
in 8 to 10 layers with desmosomes that pull cells
into spiny shapes; cells rich in RNA

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Structure of the Skin
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Cell layers
Stratum granulosum (granular layer)—
cells arranged in two to four layers and
filled with keratohyalin granules; contain
high levels of lysosomal enzymes
 Stratum lucidum (clear layer)—cells filled
with keratin precursor called eleidin;
absent in thin skin
 Stratum corneum (horny layer)—most
superficial layer; dead cells filled with
keratin (barrier area)

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Structure of the Skin
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Epidermis (cont)
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Epidermal growth and repair
 Turnover or regeneration time refers to
time required for epidermal cells to form
in the stratum basale and migrate to the
skin surface—about 35 days
 Several hormones support normal
growth and repair of the epidermis:
epidermal growth factor (EGF), insulinlike growth factor 1 (EGF-1), and growth
hormone (GH)
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Structure of the Skin
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Epidermal growth and repair (cont)
 Shortened turnover time will increase the
thickness of the stratum corneum and
result in callus formation
 Normally 10% to 12% of all cells in
stratum basale enter mitosis daily
 Each group of 8 to 10 basal cells in
mitosis with their vertical columns of
migrating keratinocytes is called an
epidermal proliferating unit, or EPU
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Structure of the Skin
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Dermopidermal junction (DEJ)
A basement membrane, with unique
fibrous elements, and a polysaccharide
gel serve to “glue” the epidermis to the
dermis below
 The junction serves as a partial barrier to
the passage of some cells and large
molecules
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Structure of the Skin
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Dermis
Sometimes called “true skin”—much
thicker than the epidermis and lies
beneath it
 Gives strength to the skin
 Serves as a reservoir storage area for
water and electrolytes
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Structure of the Skin
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Dermis (cont)
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Contains various structures
Arrector pili muscles and hair
follicles (Figure 7-5)
 Sensory receptors (Figure 7-6)
 Sweat and sebaceous glands
 Blood vessels
Rich vascular supply plays a critical
role in temperature regulation
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Structure of the Skin
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Dermis (cont)
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Layers of dermis
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Papillary layer—composed of dermal
papillae that project into the epidermis;
contains fine collagenous and elastic fibers;
contains the dermoepidermal junction; forms
a unique pattern that gives individual
fingerprints
Reticular layer—contains dense, interlacing
white collagenous fibers and elastic fibers to
make the skin tough yet stretchable; when
processed from animal skin, produces
leather
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Structure of the Skin
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Dermis (cont)
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Dermal growth and repair
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The dermis does not continually shed and
regenerate itself as does the epidermis
During wound healing, fibroblasts begin
forming an unusually dense mass of new
connective fibers; if not replaced by
normal tissue, this mass remains a scar
Cleavage lines (Figure 7-7)—patterns
formed by the collagenous fibers of the
reticular layer of the dermis; also called
Langer’s lines
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Structure of the Skin
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Hypodermis
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Also called the subcutaneous layer
or superficial fascia
Located deep to the dermis; forms
connection between skin and other
structures
Not part of the skin
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Skin Color
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Melanin
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Basic determinant is quantity, type,
distribution of melanin
Types of melanin
Eumelanin—group of dark brown
(almost black) melanins
 Pheomelanin—group of reddish and
orange melanins

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Skin Color
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Melanin (cont)
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Melanin formed from tyrosine by
melanocytes (Figure 7-8)
Melanocytes release melanin in
packets called melanosomes
 Melanosomes are ingested by
surrounding keratinocytes and form
a cap over the nucleus
Albinism—congenital absence of melanin
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Skin Color
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Melanin (cont)
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Process regulated by tyrosinase,
exposure to sunlight (UV radiation), and
certain hormones, including
melanocortins (ACTH, a-MSH) and ET-1
(Figures 7-9 and 7-10)
Cumulative effects of UV exposure may
produce age spots (Figure 7-11)
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Skin Color
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Other Pigments
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Beta-carotene (group of yellowish
pigments from food) can also
contribute to skin color
Lipofuscin—accumulates in cells that
have ceased mitosis in aging skin,
producing brown-yellow age spots
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Skin Color
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Other Pigments (cont)
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Hemoglobin—color changes also occur as a
result of changes in blood flow
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Redder skin color when blood flow to skin
increases
Cyanosis—bluish color caused by darkening of
hemoglobin when it loses oxygen and gains
carbon dioxide (Figure 7-12)
Bruising can cause a rainbow of different colors
to appear in the skin (Figure 7-13)
Other pigments—from cosmetics, tattoos, bile
pigments in jaundice (Box 7-4)
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Functions of the Skin (Table 7-2)
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Protection
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Physical barrier to microorganisms
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Barrier to chemical hazards
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Reduces potential for mechanical trauma
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Prevents dehydration
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Protects against excess UV exposure
(melanin function)
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Functions of the Skin
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Surface film
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Emulsified protective barrier formed by mixing
of residue and secretions of sweat and
sebaceous glands with sloughed epithelial cells
from skin surface; shedding of epithelial
elements is called desquamation
Functions
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Antibacterial, antifungal activity
Lubrication
Hydration of skin surface
Buffer of caustic irritants
Blockade of toxic agents
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Functions of the Skin
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Surface film (cont)
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Chemical composition
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From epithelial elements—amino
acids, sterols, and complex
phospholipids
From sebum—fatty acids,
triglycerides, and waxes
From sweat—water, ammonia, urea,
and lactic acid and uric acid
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Functions of the Skin
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Sensation
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Somatic sensory receptors detect stimuli that permit
us to detect pressure, touch, temperature, pain, and
other general senses
Flexibility
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Skin acts as a sophisticated sense organ
Skin is supple and elastic, thus permitting change in
body contours without injury
Excretion
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Water
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Urea/ammonia/uric acid
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Functions of the Skin
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Hormone (vitamin D) production (Figure
7-14)
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Exposure of skin to UV light converts 7dehydrocholesterol to cholecalciferol—a
precursor to vitamin D
Blood transports precursor to liver and
kidneys where vitamin D is produced
Process and end result fulfill the necessary
steps required for vitamin D to be classified
as a hormone
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Functions of the Skin
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Immunity
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Phagocytic cells destroy bacteria
Epidermal dendritic cells trigger helpful
immune reaction working with “helper T
cells”
Homeostasis of body temperature
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To maintain homeostasis of body
temperature, heat production must
equal heat loss; skin plays a critical role
in this process
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Functions of the Skin
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Homeostasis of body temperature
(cont)
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Heat production
 By metabolism of foods in skeletal
muscles and liver
 Chief determinant of heat
production is the amount of
muscular work being performed
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Functions of the Skin
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Homeostasis of body temperature (cont)
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Heat loss—approximately 80% of heat
loss occurs through the skin; remaining
20% occurs through the mucosa of the
respiratory, digestive, and urinary tracts
(Figure 7-15)
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Evaporation—to evaporate any fluid, heat
energy must be expended; this method of heat
loss is especially important at high
environmental temperatures when it is the only
method by which heat can be lost from the
skin
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Functions of the Skin
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Homeostasis of body temperature (cont)
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Heat loss (cont)
Radiation—transfer of heat from one object to
another without actual contact; important
method of heat loss in cool environmental
temperatures
 Conduction—transfer of heat to any substance
actually in contact with the body; accounts for
relatively small amounts of heat loss
 Convection—transfer of heat away from a
surface by movement of air; usually accounts
for a small amount of heat loss

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Functions of the Skin
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Homeostasis of body temperature (cont)
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Homeostatic regulation of heat loss (Figure
7-16)
Heat loss by the skin is controlled by a negative
feedback loop
 Receptors in the hypothalamus monitor the
body’s internal temperature
 If body temperature is increased, the
hypothalamus sends a nervous signal to the
sweat glands and blood vessels of the skin
 The hypothalamus continues to act until the
body’s temperature returns to normal

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Appendages of the Skin
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Hair (Figure 7-17)
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Development of hair
Distribution—over entire body except palms
of hands and soles of feet and a few other
small areas
 Fine and soft hair coat existing before birth
called lanugo
 Coarse pubic and axillary hair that develops
at puberty called terminal hair
 Hair follicles and hair develop from
epidermis; mitosis of cells of germinal matrix
forms hairs

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Appendages of the Skin
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Hair (cont)
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Development of hair (cont)
 Papilla—cluster of capillaries under
germinal matrix
 Root—part of hair embedded in follicle
in dermis
 Shaft—visible part of hair
 Medulla—inner core of hair; cortex—
outer portion
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Appendages of the Skin
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Hair (cont)
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Appearance of hair
Color—result of different amounts, distribution,
types of melanin in cortex of hair (Figure 7-18)
 Growth—hair growth and rest periods
alternate; hair on head averages 5 inches of
growth per year
 Sebaceous glands—attach to and secrete
sebum (skin oil) into follicle
 Male pattern baldness (androgenic alopecia)
results from combination of genetic tendency
and male sex hormones (Figure 7-19)

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Appendages of the Skin
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Nails (Figure 7-20)
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Consist of epidermal cells converted
to hard keratin
Nail body—visible part of each nail
Root—part of nail in groove hidden by
fold of skin, the cuticle
Lunula—moon-shaped white area
nearest root
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Appendages of the Skin
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Nails (cont)
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Nail bed—layer of epithelium under nail body;
contains abundant blood vessels
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Appears pink under translucent nails
Nails may have pigmented streaks (Figure 7-21)
Separation of a nail from the nail bed is called
onycholysis (Figure 7-22)
Growth—nails grow by mitosis of cells in statum
basale beneath the lunula; average growth about
0.5 mm per week, or slightly over 1 inch per year
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Appendages of the Skin
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Skin glands (Figure 7-23)
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Two types of sweat glands
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Eccrine glands
 Most numerous sweat glands; quite
small
 Distributed over total body surface with
exception of a few small areas
 Simple, coiled, tubular glands
 Function throughout life
 Secrete perspiration, or sweat; eliminate
wastes and help maintain a constant
core temperature
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Appendages of the Skin
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Two types of sweat glands (cont)
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Apocrine glands
Located deep in subcutaneous layer
 Limited distribution—axilla, areola of
breast, and around anus
 Large (often more than 5 mm in
diameter)
 Simple, branched, tubular glands
 Begin to function at puberty
 Secretion shows cyclic changes in
female with menstrual cycle

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Appendages of the Skin
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Skin glands (cont)
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Sebaceous glands
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Secrete sebum—oily substance that
keeps hair and skin soft and pliant;
prevents excessive water loss from skin
Lipid components have antifungal activity
Simple, branched glands
Found in dermis except in palms and
soles
Secretion increases in adolescence; may
lead to formation of pimples and
blackheads
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Appendages of the Skin
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Skin glands (cont)
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Ceruminous glands
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Modified apocrine sweat glands
Simple, coiled, tubular glands
Empty contents into external ear canal
alone or with sebaceous glands
Mixed secretions of sebaceous and
ceruminous glands called cerumen (wax)
Function of cerumen to protect area from
dehydration; excess secretion can cause
blockage of ear canal and loss of hearing
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Cycle of Life: Skin
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Children
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Skin is smooth, unwrinkled, and
characterized by elasticity and
flexibility
Few sweat glands
Rapid healing
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Cycle of Life: Skin
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Adults
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Development and activation of
sebaceous and sweat glands
Increased sweat production; can
result in body odor
Increased sebum production; can
result in acne
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Cycle of Life: Skin
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Old age

Decreased sebaceous and sweat
gland activity
 Wrinkling (Figure 7-24)
 Decrease in body's ability to
cool itself
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The Big Picture:
Skin and the Whole Body
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Skin is a major component of the
body’s structural framework
Skin defines the internal
environment of the body
Primary functions are support and
protection
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